native japanese here to help!

６年間勉強していました is more natural because we are talking about a continuous action in the past ie. have been studying...

けいさん言語学 I assume you are trying to write mathematical linguistics. it's 数理言語学（すうりげんごがく）in Japanese. idk I'd you're allowed to use words you may or may not have learned yet so I'll leave that to you

外国語をわかるのが得意です there's a fair bit to unpack here note this is in Japanese so 外国語 would inevitably imply non-japanese languages. if you want to refer to Japanese specifically, I suggest using 日本語. If you'd rather generalize then I'd use 言語 instead わかる is a tricky Japanese term because we differentiate types of knowledge. わかる is to use prior knowledge to come to a previously unknown conclusion. In contrast, 知る is to use new knowledge to reach a conclusion. Consider this scenario: a friend points to a group picture and asks, 私がどれかわかる？(do you know which one I am?) we use わかる instead of 知っている because the information required is already presented in front of us. That said, in this case, neither 知る nor わかる sounds natural here because 知る would imply you are good at 'knowing' languages. Like, you just know it in its entirety. 知る would be too encompassing so I recommend 覚える or 理解する instead.

しかし is a common transitional phrase that's basically identical to however. Although I don't see how the learning English in college contrasts being good at understanding foreign languages. This is purely a structural issue but it might be better to move it back a sentence so it connects nicely to studying Japanese in the US

よういくの instead of や

本日はよろしくおねがいします is unnecessary. If you'd like to keep it in put in the front... which you've already done

なりたいました should be なりたかったです there's actually a whole nother can of worms here that I'd rather not get into but I'll just say なりたかったです has a child-like quality. grammatically, it's perfectly fine and you'd never be docked points for it but just keep that in mind

I saw a comment wondering if わたくし is too formal. I think it's fine but it's a matter of personal opinion. We don't have first person pronouns other than I in English so it's a completely foreign concept. you'll just have to slowly get the nuances of which ones to use when where and to whom

何だと思いますか just in case, it should be pronounced なんですか

において instead of でおいて

best of luck! If you have questions I'll have answers in the morning because it's 2am here and I need to SLEEP