Name of language of translation
(Proper answer from my comment in OP's question)

My proposal:

getLanguageNameInNativeLanguage

Other than that, there's a term for that: Endonym. See wiki: https://en.wikipedia.org/wiki/Exonym_and_endonym

Quote:
An endonym or autonym is an internal name for a geographical place, or a group of people, or a language or dialect.

For instance, "Germany" is the English language exonym, "Allemagne" is the French language exonym, and "Deutschland" is the endonym for the same country in Europe.

getLanguageEndonym() or something similar would look prettier, but a layman of the app doesn't understand it. If language is something important in the app, I would use the "academic" term.