There's a button for iNkscape Preferences which is 2nd in from the right on the command bar (just below menu bar). Then Interface should be 4th from the bottom in the list on the left side of Inks Prefs. Then Language (requires restart) is the first option at the top (on the right side of Inks Prefs). Beside that is a dropdown menu with lots and lots of languages. "System default" is the first one, and what normally shows up as the default. I don't know how yours got changed.