You actually can import words! It has a super simple format of just "word,translation" that you can copy paste in, and it'll automatically assign them all as Tier 1. You can also export and import progress to any device using a .json file.

And yeah, it would barely take any effort to make it language neutral. But since this was a project for my own uses, I didn't see the point. But if the community gets large enough and people are interested, I'd be happy to make those tweaks.