You can speak another language instead of English in a speedrun video?

Yes

Specifically - it shouldn't matter in the slightest, but be sure to check the actual game rules for any game you are deciding to run. Some do have specific rules in how they want runs recorded.

I’ve never seen a leaderboard that requires you to speak at all during the video. The only issue I can think of would be if your voice covers the game audio in a game that requires that, though that’s irrespective of what language you’re speaking.