I recorded a run in all language settings (but russian, technical issues), cut everything but the text speeches and here's the result:
In any%, english is the fastest and other than having to type different words in the texts prompts, one notable difference in gameplay is that you don't have to rebuild the hidden option to get the laser in any other language than english.
When going for individual levels, here's the best languages: Intro: Polish / Turkish (turkish is slower by milliseconds) Act 1: Danish Act 1 Boss: Doesn't matter Act 2: ??? Act 2 Desktop: ??? Act 2 Boss: English Act 3: English Act 3 Desktop: Turkish Act 3 Boss: English Final: Doesn't matter
Edit: It also seems that it's not worth it to change language mid-run. Changing languages would need to take less than 2 seconds. Here's my Shortest Path Solver spreadsheet to show that. https://www.dropbox.com/s/4dn2aq7m1mxb0uk/Pony%20Island%20Shortest%20path%20solver.xlsx?dl=0 If you wanna tinker with it, you'll need the https://opensolver.org/ add-in as there are too many cells for Excel's built-in solver.
Keep in mind that changing language boots you back to the title screen, so don't try doing that during a run
