Usually if the historian doesn't read the language (unless you're doing oral history there's no need to demonstrate spoken proficiency), the next ideal solution would be to either hire or partner with someone who can read the documents in their original.

While some historians do use documents in translation, this as a practice is frowned upon as, once a document is translated, it's not really a primary source anymore (the translator has to make decisions about word choice, where to break sentences, syntax, etc -- look at religious studies where people get into fights over prepositions and vowels).

Most (if not all) universities require students to demonstrate competence in the language(s) they'll need to conduct research for their dissertations; usually it's when you get more advanced and expand outward that you run into this issue -- and that's where and how partnering with someone else is probably the best option.