One of the strengths of Free Software is to provide users with software in their language. More translations for MuPDF will in the middle and long term bring more users to try it. FSFE has already asked its translators-team for help on this matter. http://pdfreaders.org/index.en.html https://wiki.fsfe.org/PDFreadersTranslations
We don't have the time to provide and maintain language translations for MuPDF. If someone steps up to take on the task, then they're welcome to do so, but until such a time I will consider this a won't fix issue.