We need 2 language tags minimum to indicate the known language, and willing to learn language.
For many questions, I disagree with you. Looking at the questions from the definition phase, for very few the language the person asking already speaks are even relevant. That can be indicated in the question if needed, I see no need for a tag for that.
The way the site is defined, not all questions will even be about a language - it's also about how to learn languages in general.
What's the plan of creating all 6.5k language tags? I meant, suppose if we start with 10(or 25 or 50...) popular language tags, and the user wants to ask a question about a language that doesn't have a tag(new user or with less reputation), they can't create new tags right?
You can't, as far as I know, just create a tag without a question for it anyway, and tags are even deleted if there's only one question with it after some time. So there can be no plan anyway.
It doesn't really matter that much, I think, though. It's basically a question every site on Stackexchange faces and every site somehow deals - if someone wants to ask a question for a programming language that doesn't have a tag yet on StackOverflow, they also face that problem. If you're asking about an obscure TV series on Movies & TV, there will be no tag yet, either.
Usually, there should be at least one other tag that's appropriate, about the subject instead of the language. So "vocabulary" or "grammar" or "resources" or whatever the community decides on. If there isn't, what happens on other SE sites is usually that the question asker just chooses some kind of tag and someone with the appropriate privileges then edits and creates the tag. It's really not that much of a problem.