There is a community driven effort to localise Kanji Tree's user interface into various languages. Currently, only languages which use Roman, Greek, or Cyrillic alphabets are supported. I would like to implement support for other scripts, but realistically it's not going to happen unless and until there's sufficient demand.
Users can add translations for their preferred language into this Google Sheets spreadsheet:
I take occasional drops of new translations into the app, if they're looking reasonably stable.
Kanji Tree contains two dictionaries, EDICT for words, and KANJIDIC for kanji meanings. These dictionaries are Japanese to English, which is the reason the games only use English words.
There is a more recent version of EDICT, called JMDICT, which also contains translations for some other popular languages, and I have a version of Kanji Tree which uses that. It's not yet ready for public release, but if I can get it a bit more polished then I'll release a Pro version which uses your preferred language if available, and English for anything that's missing.
Compared to EDICT, JMDICT is huge! There's no way I'll be able to squeeze JMDICT small enough to get it into the free version, so that will always just be English only. Download size is absolutely critical, when it comes to encouraging people to install free software, whereas people are much more willing to accept large installs if they've paid for it.