I know this isn't the usual type of question that is often asked here but you historians have to have gone through this one time or another. I'm doing a project about one of Napoleons sieges and I cant find a way to find any of his letters he wrote leading up to it. If you go to the French national archives it's in French. Even if you translate what you need to find into French it wont register. If you guys have any pointers it would be very much appreciated.
If you don't speak the target language then your best bet is an edited collection of primary sources. Often, historians will collect significant or interesting primary sources, translate them and publish them with a commentary. There are also online databases for various collections of primary sources which you can search for. With regards to Napoleon's letters, as they are a significant source it's quite likely that they'll have been translated or digitised somewhere, it's just a matter of finding the right place.
How did you hear about the letter you're looking for? If it was from a text with citations, you should be able to get a French title you can search for in those archives, the name/publication info of a translation, or a secondary source that cites the primary source or translation.
I haven't read it, but as /u/Abrytan and /u/b1uepenguin have said, this might be one such collection of Napoleon's letters that is worth looking into: https://www.amazon.com/Letters-Napoleon-J-M-Thompson-ebook/dp/B00BPW44RC/ref=tmm_kin_swatch_0?_encoding=UTF8&qid=1544845143&sr=1-1
As a more general proposition in terms of finding such sources, in some countries it can be extremely difficult, and in some cases dangerous. The recent criminal trial of Matthew Hedges or the apparent police extrajudicial killing of Giulio Regeni are just two recent examples the threats academic researchers face in the Middle East.
In many such countries archives remain decidedly closed even for events that took place 50-100 years ago and even something as basic as the contemporary demography of a country can be a critically sensitive political taboo.
The result in many modern histories of the Middle East is that the sourcing in English, French, Hebrew, and to a much lesser extent Turkish can be very good, but the number of sources in Arabic or Persian may be very poor.