In my opinion, a bilingual documentation should be just fine. For ease of use (and to prevent surprises), i'd specify right at the beginning that "the document is available in the following languages" and maybe state in a sentence or two where to find which language.

It seems you technically don't even need to use proper english or characters that are used in any other language, thanks to the ability of UTF8 to combine stuff. I'm looking at you, Acme::Umlautify

But in your case, making sure that english is one of the languages in the documentation would be very appreciated. Yes, the final use case might be limited to a certain region or language. But always think of the person that is hired to do the actual implementation of a project that uses your library (or, for that matter, maintain the software in years to come). They might be from a completely different part of the world.