I take it for granted that all human beings (aside from those with serious neurological impairments) are able to think. Now if you think right now it's most likely that you'll be thinking in your first language. But is the ability to speak really essential to be able to think? After all it is, as some people would say, talking in your head. But I don't think that that is the case.
Imagine if there was some big calamity that wiped out a whole significantly large area apart from one baby. Now also imagine, through some string of increasingly unlikely events, that the baby survives and eventually becomes a fully-grown male. Don't you think that this fully functioning adult would be able to think? He'd have no reason to learn to talk as he'd never have to communicate with any other humans, but why would he not need to think? He may notice dark clouds forming in the sky and mentally plan a day that will ensure that he remains in places that'll be moderately dry. But how can you mentally plan without being able to think.
As such, I believe that there is a universal human language known innately to all members of that species. We get conditioned when we learn a language so that we think using that language rather than the Universal Language which we would think with before we learn to talk. I imagine it would be rather different from current spoken language as it would never actually have to be spoken, merely thought, and so not limited by vocal capabilities. But it's all just a theory, most likely flawed due to my lack of scientific knowledge, but nonetheless, as I said earlier, I hope you've enjoyed reading this.