For example, I know that we don’t know a whole lot about what the Achaemenid Persians did in the east of their empire compared to anything they did near the Greeks. Do the Iranians or Pakistanis or Indians have a better idea of what happened in the eastern Persian Empire or what the Persian kings did because they have access to primary or secondary sources that haven’t been translated into English? Is this a significant issue for people doing history - that the sources may be there, but aren’t accessible because they would be either too expensive to translate or too difficult to locate without knowing the language? Or is there a lot of cooperation between historians in different countries?
There's a couple ways to address your question, and it raises some historiographic points. First, there are an infinite number of primary sources unavailable to English speakers if they only speak English, just like there is an infinite number of English primary sources unavailable to someone who does not speak English. Even English sources can be unavailable just because the handwriting is illegible. There will always be sources that exist but historians can't access or use.
Second, any historian working on a subject probably knows the language of their sources. PhD programs require historians to learn a language, although some don't require it for students whose area of study is predominately English. For example, I'm working on my PhD in early American history so I'm not required to learn a language, but if I look beyond the English colonies, I really should pick a language. I'm still a couple years from needing to make that decision. Some historians know a lot of languages- Sam White at OSU knows at least English, Spanish, French, and Turkish.
Third, theres a few reasons why non-western history might not be covered, and it's likely not entirely the language barrier. It could be that the sources don't exist or that archives aren't organized where historians can find them. Also, historians in western countries tend to prioritize western countries and perspectives so as a discipline, we probably haven't asked the questioned needed to bring some primary sources to the surface.
Fourth, historians might have looked at the sources to answer questions, but historians aren't obsessively transcribing and translating. Someone might have come across a source in an archive, translated it to English in their notes, saw it didn't answer their research question, and never looked at their note on it again. Our discipline is trained to look for answers to questions we ask. Generally, we aren't finding undiscovered primary sources- they're documents other historians have looked at before us. But historians are looking for things when we travel through archives and read sources. If it doesnt address our question, we might not do anything with it. It's how history works- think about how many Thomas Jefferson biographies there are. Historians keep looking at the same material and asking new questions to have new conversations about the sources. It might just be that some non-English sources are answering questions no one asked about yet in English.