I want to stream a game for my friend to watch me, and I would be talking in spanish while playing. Is that allowed? Can I talk in spanish and the speedrun can still count as valid?

yes

Yes, though some games may require game audio to be part of the run, so make sure you check the rules.

I don't wish to shame an honest question, questions are how we learn, but....what made you think the answer to this could possibly have been no?

Well, the reason why I ask this is that many communities are really toxic and do not allow speaking in any other language that isn't spanish.

I have never heard of that, but if there is a game/community out there rejecting runs for the language the runner speaks, that should be reported to site staff right away.

I didn't mean speedrun.com community, but in general other communities.

It's important to consider that speedrunning is still a solo activity. When you stream, you're in control of your content; you're not speaking directly to other members of the community. Language rules make a little sense when the most important part of a community is communication, but with speedrunning, the most important part is the run of the video game.