We didn't have to specialize in a field in my undergraduate history program. We had to take a certain amount of prerequisite classes (various survey classes - modern Europe, British, Canadian, American history), and then pretty much whatever other lectures and seminars we wanted. I was originally planning on studying Canadian history, which is local history for me, and since the scholarship is in English and French there's no language barrier. But I kept being drawn to medieval classes...medieval Europe, medieval England, the crusades, a class about Byzantine history over in the classics department.

I wanted to keep studying in grad school, so I applied for both Canadian and medieval programs. I got in to a Canadian history program where I would have had full funding, a teaching position, and an advisor right away. But I also got in to a medieval studies program where I would have no funding, no teaching job, and no advisor. Everything was much more chaotic, I'd have to learn several medieval languages, and I'd have to find something to research and someone to advise me on my own. Naturally, I chose the chaos.

Your university will be able to answer questions about quotas or other requirements much better than we can, since every university is different and even various departments in the same university could be completely different. Language barrier probably doesn't matter since you'll have the chance to learn the languages you need as part if your degree (well, ideally...I mean, it would be unusual to have a Japanese history program without any Japanese language classes...).

I feel like I was probably in a similar situation, at least in the sense of wanting to study two wildly different things, and everything worked out ok. But definitely check with the university too, academic advisors are there to help with this kind of stuff!

If you feel like you are torn between completely different specialisations, that is completely fine. u/WelfOnTheShelf has already provided one such story and I can second that. That's absolutely natural at this stage of your academic journey. Even many senior scholars may change their areas of specialisation over time. Peter Laslett was an exemplary intellectual historian and then became a historian of 'population and social structure' later in his career, though I'll never understand how a sane person could leave intellectual history for counting the number of kids some farmers in a town had... (Just kidding, social historians! :P)

There was a time when I wanted to do Turkic history; Göktürks, Khazars, etc. Looking back now, I can see some common threads between now and then. I was interested in things like why Khazars converted to Judaism, if they developed some theology or philosophy of their own within Judaism, if Göktürks were influenced by Buddhism... Briefly, I even toyed with the idea of learning Tibetan somehow to see the relationship between Tibetan Buddhism and Göktürks' mythology. But besides this interest in intellectual history defined very permissively, almost everything is different. The languages required differed and so did the job prospects. That kind of a specialisation required Russian, Chinese, but I ended up going for modern Turkish intellectual history and learning Italian to study intellectual encounters between Italy and Turkey.

The language barrier is not too much of a problem at an early stage. As u/WelfOnTheShelf points out, if you join a Japanese studies programme (things may differ in PhD programmes depending on the country, though), they will likely have some sort of preparation for students without much prior knowledge of Japanese. That said, I think it is a good idea to think about the languages you will have to master in advance. Unfortunately, finding a job in academia might be tough nowadays. So it might be worth asking oneself 1- Do I have the determination and means to persist through all this uncertainty, and 2- If I may change my field, how could the language skills of this field help me? Some language skills might be useful in other jobs, while some might be quite useless for anyone other than historians. This is not to say we should all learn Chinese and coding and become historians of modern China, just in case we cannot land an academic job and end up having a translation job in a Chinese company! It is just worth thinking about that, especially if there is some uncertainty about whether one will and can persist through the vagaries of the academic job market.

My own choice was largely determined by the shift in my interest towards modern Turkey. But I would be lying if I told you I did not consider future job opportunities. This is something that will widely vary depending on the case, university, and country. Where I live, if there is one position for a historian of the Khazars, there are ten positions for historians of modern Turkey. That was a consideration, though I cannot imagine anyone merely choosing their field because it is easier to find a job through it.

If I were in your position, I would try to consider these issues of the language barrier and future job prospectives as calmly and objectively as possible. You can talk to academic advisors and PhD candidates to see what their experiences have been like. Then, if convinced finding a job in the field is feasible or at least not impossible, I would choose the field that excited me the most!

Good luck!