Is it worth it to play on other languages that aren't English? Obviously Japanese is the fastest but would there be any significant difference in textboxes between English and some other language like spanish or German?

There is a European PAL version, but boomer skip and GBS doesn't work on PAL iirc, so any text timesave you could get from other languages aren't worth it. If you play on console, those are valid versions that can be submitted to speedrun.com :) The version differences shouldn't matter until you get to the top level.

Pal has some Changes of Enemies as well. The Ice Cherbill will shoot way faster, which makes Ice Cherbill skip impossible as well.