Can any historians inform me about the extent of this problem, and what is being done to broaden our historical horizons so to speak?

It is an endemic problem, and one caused by any number of factors, from language barriers through chauvinism to politics. There isn't going to be any answer as to "how much", simply because such a calculation would be essentially impossible, nor would simply listing examples be all that constructive, but I will use one example which I think illustrates a number of the historiographical issues that are in play here, borrowing from an answer I wrote some time back that was specifically about historiography of the Eastern Front in World War II, with some slight modifications related to what you are asking here.

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For a long time, the study of the Eastern Front in World War II was significantly hampered by all of the factors that I mentioned above, and more beyond that. Much of the core issue stemmed from the rising antagonism between East and West which meant that Western historians lacked access to Soviet archives and other sources, while at the same time they did not only have extensive access to German sources, they further had access to the Germans themselves. this is perhaps best exemplified by Gen. Franz Halder, who had served as head of OKH during the war, and after became closely involved with the U.S. Army Historical Division, although the memoirs of commanders such as Manstein, Guderian, and Mellenthin too were hugely influential in shaping post-bellum perceptions of the Eastern Front in the West. That isn't to say that there weren't always Western scholars doing their very best to seek out an honest, and balanced understanding, but sheer will-power isn't enough to overcome something such as an archive closed off by the Soviet government.

As the US and her allies began looking towards West German rearmament, there was political expediency in this acquiescence to German perspectives, which included veritable whitewashing of Wehrmacht involvement in war-crimes (The "Clean Wehrmacht" myth), and also a denigration of Soviet military capabilities, denying that the Red Army had won through anything other than sheer numbers and the leaderships' ruthless willingness to use them (A quick note is worth mentioning here, namely that it is fair to say that the Soviets did engage in such attacks at points, especially in the early days when things were collapsing, and even more so with the untrained civilian levies that were raised and barely armed in desperate delaying actions. The issue isn't whether they ever did it, but whether it characterized the typical Soviet attack and general sense of tactics for the war. Several tropes are addressed here). In "When Titans Clashed" Glantz and House set the tone of earlier historical study - and the shift of which they were riding one of the first waves - in their introduction:

For decades, both popular and official historians in the West presented the Soviet-German struggle largely from the German point of view. As a practical matter, German archives and memoirs have been readily available as sources about this struggle since the 1950s, whereas their Soviet equivalents were obscured by difficulties of ideology, access, and language. Even when published in translation, most popular Soviet accounts of the war were filled with obligatory communist rhetoric that made their factual assertions appear to be so much propaganda. Westerners quite naturally viewed with suspicion the many detailed Russian-language accounts of the war and the few Western studies that relied on them.

Prime examples of this can be found in the works produced under Halder's supervision, studies that were nominally US Army publications, but in many ways apologia for German military skill, defeated only by insurmountable numbers. In their work "The Myth of the Eastern Front", Davis and Smelser provide some choice quotations used in describing the Soviets:

The Slav psyche - especially where it is under more or less pronounced Asiatic influences - covers a wide range in which fanatic conviction, extreme bravery, and cruelty bordering on bestiality are coupled with childlike kindliness and susceptibility to sudden fear and terror.

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The Semi-Asiatic [Russian soldier] possesses neither the judgement nor the ability to think independently. He is subject to moods, which to a Westerner are incomprehensible; he acts by instinct. As a soldier, the Russian is primitive and unassuming, innately brave but morosely passive when in a group.

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The greatest patience and endurance of suffering, a certain inertness and submissiveness to life and fate, lit tit· initiative, and in many of them [...] an easily aroused inclination toward cruelty and harshness which may be considered as part of the Mongol heritage in view of the good-natured disposition of the people.

Similar commentary comes from the aforementioned memoirs, well characterized by translator Steven Newton's introduction to Gen. Raus' memoirs which he describes as "very much a Cold War period piece, in which the Germans fought hard but honorably against the malevolent Soviet hordes." Raus, speaking tactically, describes Soviet attacks as "waves upon waves" at a number of points, and with this passage being a good exemplifier of this characterizations:

Thousands of Red Army soldiers filled the snowfields, slopes, and depressions on the endless steppes. No German soldier had ever seen such multitudes advance on him. The leading ranks were thrown to the ground by a hail of high-explosive shells, but more and more waves followed. Every attempt on the part of the Russian masses to reach our lines was thwarted by the fire of machine guns, artillery, and heavy weapons. The frontal assault had been halted in its tracks.

This is quite similar to other commanders works, such as here in Mellenthin, with passages such as:

Right up to the end of the war the Russians did not bother to loosen up their attacking waves and sent them for­ ward almost shoulder to shoulder. The herd instinct and the inability of lower commanders to act for themselves always resulted in densely packed attacks. Thanks to superiority in numbers, many great and important succes were achieved by this method. However, experience shows that it is quite possible to smash these massed attacks if they are faced by adequate weapons handled by trained men under determined commanders.

And of course not to mention Manstein's "Lost Victories", which was a hugely influential work in the West:

In the days following its arrival at the Mishkova on 19th December, the relieving army had become imbroiled in heavy fighting against the never-ending waves of forces thrown in by the enemy from Stalingrad to halt its advance. Despite this, 57 Panzer Corps had succeeded in gaining a foothold on the north bank of the river and, after a series of ding-dong engagements, in forming a bridgehead there. Mass attacks by the enemy brought him nothing but bloody losses.

Manstein's impact is probably illustrated no better than with the praiseful foreward to the work provided by B.H. Liddell-Hart, the British historian who similarly venerated Rommel and generally pushed the "Clean Wehrmacht" narrative, where he wrote of Manstein as "the Allies' most formidable military opponent - a man who combined modem ideas of mobility with a classical sense of manoeuvre, a mastery of technical detail and great driving power."

And although less influential, nevertheless illustrative of the German characterizations, this comes from Kurt Meyer's memoir of his time in the Waffen-SS:

Out of the gray light of dawn came masses of Russian infantry who rushed the position singing and yelling. The foremost ranks had linked arms, thus forming a continuous chain which stamped across the ice in time to the wild singing. Mines tore great holes in the ice cover, forcing the Soviets to break their chain. But the mines could not stop the roused mass rushing my comrades like a machine. The Soviets were caught by our fire in the middle of the river and laid out on the ice like ripe corn under the swing of the scythe.

My soldiers lost faith in God and mankind as the succeeding Russian units came clambering over the fallen Red Army soldiers and continued the assault. The attack was being carried out by the Russian 343rd and 31st Infantry Divisions and the 70th Cavalry Division. Three newly-raised divisions on the attack against a few hundred men spread across 8,000 meters and practically alone, each left to his own devices and having to cope with this mass!

So to tie this all back to the original point, the above are examples of the primary and secondary sources that were of great influence in the West in the immediate post war years, and up through the 1980s at the very least. As David M. Glantz and Jonathan House point out, the very fact that we refer to it as "The Eastern Front" belies the perspective from which Western historiography approaches the conflict. Again, not to say that no earlier historians were attempting to give the Soviet Front its fair shake - the works of John Erikson or Earl Ziemke remain well respected, even if they have shown their age - but even the best intentioned authors were hampered by the lack of good Soviet sources and a plethora of German ones. And even putting aside the obvious biases of the German memoirists, and their almost universal desire to find something to blame defeat on other than their own shortcomings - whether it be Hitler's meddling, the Russian winter, or the "Asiatic hordes" - they also enjoyed focusing on the good times, 1941 and 1942, rather than the bad of 1944 and 1945.

I'll add a bit to u/Georgy_K_Zhukov's discussion of Soviet military history with some other parts of Russian/Soviet/Eurasian historiography (for reference, here is a lengthier discussion on the subject I wrote a while back here).

One thing alluded to in the military history answer is just how striking the paucity of sources were that historians had to work with until circa 1990. Whatever a historian's opinion on the Cold War - and opinions mattered a lot - basically if you were writing political or social history of any kind, you pretty much had to work with: 1) official Soviet publications, 2) dissidents' accounts published in the West, 3) the Smolensk Party Archives that were captured by the Germans and then captured by the Americans in World War II, and 4) a sociological study conducted by Harvard on Soviet immigrants in the 1950s.

Even within these area studies, Central Asia can be a particular afterthought even for academic historians. I think the number of academic historians who research and publish works on modern Central Asian history in English could be counted on two hands. Which is a little crazy because Russian and Soviet historians, anthropologists and archaeologists have written boatloads on the region for decades, if not centuries. The idea that Western academic historians would focus on these non-Russian areas has only really started to become a thing in the past 15 years or so, with Eurasian studies and nationalities studies.

I think the worst offending example of how this comes across in popular history would be Peter Hopkirk, who wrote The Great Game, as well as some other volumes on British spycraft in Central Asia. The main characters are always the British gentlemen spies, and their adversaries are always villainous Russians, and actual Central Asians (or the many South Asians assisting the British in their work) are essentially just an exotic, nameless local color. Americans (and British too) can be really, really bad in actually trying to see historic events from the points of view of other peoples. An example along these lines that came to my mind a few days ago is Yeveny Primakov, who was a Soviet intelligence officer who served as Chairman of the USSR, as well as Foreign Minister and Prime Minister for Russia in the 1990s, and was an Arabist. As far as I can tell, the US has never had someone serving that high in government who had as deep a knowledge of Middle Eastern history or who is fluent in Arabic, despite all the blood and treasure the United States has expended in that region.

Anyhow, I'd like to also take things closer to home - how Americans understand American history is still a very white Anglo-centric phenomenon. I'd like to highlight this fantastic answer by u/anthropology_nerd as a corrective to a question kind of along these lines, because there is still a tendency to equate "America" with "white Anglo settlers and their descendents", and negate pretty much every other population involved, Native or black. This is admittedly more of a popular history phenomenon than an academic history phenomenon, but considering its so close to home I think it's especially troubling. Like the fact that Cahokia probably had a bigger population than London did at the time, but almost no Americans have even heard of the former city, has real consequences, as it means an extensive archaeological site has been bulldozed up until very recently in order to build motels, highways and suburban sprawl.

For the Sengoku I actually don't think it's all that bad now-days, probably compared to other places and time periods. Sengoku is a popular enough topic with enough professionals who know the language that scholarships, especially recent analysis-type scholarship are all fantastic I feel. They directly cite the Japanese source, both primary and secondary, to build their theories, so things like the transition of the state-apparatus into the early modern (Edo), the relationship between different aspect of society, and even the military transition is fairly well done. Academic-level local history aside from the three unifies or the modern population centres of Japan are a bit more lacking, but that applies to both sides of the pacific and is slowly getting better too.

What's really missing in academic scholarship in English are bibliographical works of the players (other than the three unifiers) and military analysis focusing on analysing specific battles and campaigns. This means English-only audience are basically stuck with Osprey publishing stuff, which are already getting a bit old even if they were academic-level with proper source-analysis and citation, which they're not. I think a lot of Osprey publishing stuff are also basically translation, or at least taking inspiration from, Japanese pop-level history publications from a few decades ago.

So where for English publications on, say, the process of fragmentation and reconstruction of political power in the Kantō plain you find the publication citing specific law codes, specific written orders, specific letters, specific passages in diaries, or specific archaeological finds. Meanwhile, for something like the life of Takeda Shingen, the English audience is limited to decades-old publication that repeat (with no citation), popular historical stories that are at-best spiced-up and/or biased narrative history compiled in the Edo period, and at worst out-right fiction. For example, the current scholarship in Japan on the battle of Anegawa that I tried to use as a basis for BazBattle's youtube video (shameless plug) is based current Japanese research which focuses on using the letters and reports of the combatants and the very first round of compiled records to get a very rough picture of what happened. The Wikipedia article on the other hand contain a mix of things, a lot of which are based on things trained academics would reject as probably made-up (and the map in the tool-box we know is wrong).

Also a funny thing I find is that the primary sources tend to focus on detailed political history (who did what, when) since they are diplomatic documents or clan history/records, but academic is obviously focused on compiling them into a more general "cause & effect," "continuity & transition," or "general trends" picture. Which means that the primary sources are overly focused on political details and big-man, while the academic scene has almost nothing on it. It makes sense why it would be this way, but sometimes I do wish there were academic-level, sourced, cited, and fact-checked, who-did-what-when, so it's easier to just reference events. This really applies to all non-modern, non-English-speaking areas I find.

Anyways, just to close out. For Sengoku history, I find that English-language academic analysis, especially newer ones, are pretty much on-point. If you want to "understand" history, you're actually not missing much if you have access to academic-level stuff. On the other hand, things that amateurs like to "know and tell" because it's cool, which I totally sympathise with, there's basically no primary sources translated and no good analysis.

The majority of Arabic language texts have never been translated into English; that's an entire body of literature - many books on and about history included - that's largely unavailable to the anglosphere.

The vast majority of cuneiform texts have never been translated, period. Assyriology started out as a field almost wholly devoted to translation and philology.

The language barrier issue is enormous, once you start looking for it. Western historiography virtually reinvented the wheel to deal with some issues that Arabic historians encountered, addressed, and forgot about centuries ago, and in a very real way, the so-called European enlightenment - the 'rediscovery' of classical knowledge by European elites - was facilitated by and moderated through Arabic translations in the first place! Many classical Greek texts only survived second-hand through the Arabic translation movement, and it's a wonder that so many westerners have neglected to appreciate that fact, failing to turn back to the Arabic corpus to see if its representative authors had anything worthwhile to say on their own behalf.

I would like to give an example of the French. In the lead up to world war 2 the British made several moves which the French see as having undermined their resistance to the Germans.

Specifically getting France to not directly aid the Republicans in Spain, the 1935 Anglo-German Naval Agreement and of course the Munich Agreement. These are things that in English language sources are generally dealt with from a British point of view. Not that the French point of view is necessarily 'correct' or 'better' but many English speakers are unaware of how British policy undermined French resistance to the Germans and how the French feel and felt about it.

We could also look at someone like Robert Surcouf, where French and British sources are different.

Basically the French are much more aware of English language sources than we are about French sources. One can see it even on this board where people love the book peasants into Frenchmen, a great book to be sure, at the expense of French nation building 1789-1870, because this book is in English and the other books are not.

Moving on, when I was in Morocco it was hard to find a good English book about the riff war while they had many Arabic, French and Spanish books, this was surprising at the time.

If one wishes to learn about say, the privateer city of Saint Malo, the vast majority of books and articles are only in French.

In conclusion the French point of view is often ignored or unavailable in the English language. Seeing how important French is academically I assume the situation is much worse in many other languages.