I see most of the runners using the Spanish language in the game, but is there any difference from other languages?

Hello again, Mew. Yes, in a way, the game does change and can make an impact on your run. For example, the English version may have 100 characters of text in a section, meaning you're spending more time scrolling through. The Spanish version, however, may only have 75 characters of text, and can save you some time. Although you don't have to change the language of the game to get first place, it still can save you a bit of extra time... just know which one is "yes" and "no". ;)

Thanks again, but I don't think that it will change much, I'm just trying to start speedrunning. If someday I try getting a real good time, I'll change the language.

Once again, good luck. I think personally I'll stick with the English versions as well, it just seems more natural to me.

Does anyone know exactly how much time Spanish or Italian saves over English?

Only about 1 second xD

If I remember correctly, Spanish saves 1 second over English, Italian saves 1.5, and Japanese saves 2. So yeah, it's not very important, and you won't be able to switch to Japanese if your Wii U isn't from japan.