Any idea what file in my .config can I change / delete to reset language settings?

I will look into it later, but even after selecting Spanish (from Spain) as system language, several strings remain untranslated, which is weird, because I remember seeing them in Spanish before.

I don’t know what type of editing you need to do, and I haven’t used it myself, but if I’m not wrong, LibreOffice Draw can edit PDFs.