Genesis 11:1: (Sefaria English translation)
וַֽיְהִ֥י כָל־הָאָ֖רֶץ שָׂפָ֣ה אֶחָ֑ת וּדְבָרִ֖ים אֲחָדִֽים׃
And the whole earth was of one language and of one speech.
After they built the Tower of Bavel,
הָ֚בָה נֵֽרְדָ֔ה וְנָבְלָ֥ה שָׁ֖ם שְׂפָתָ֑ם אֲשֶׁר֙ לֹ֣א יִשְׁמְע֔וּ אִ֖ישׁ שְׂפַ֥ת רֵעֵֽהוּ׃
Come, let us go down, and there confound their language, that they may not understand one another’s speech.’
1 - What was the common language that they spoke? Can we assume that it was Hebrew, in some form? (I'm basing that assumption based on a Midrash that I heard in yeshiva that the world was founded on the Torah and the Hebrew letters and language existed prior to the founding of the Torah. I may not be paraphrasing the Midrash fully correctly, but that was its gist.)
2 - I must be missing something in the translation of the 2nd verse that says that a person did not understand what his neighbor was saying. Isn't the purpose of language to be able to communicate with your neighbor? If your neighbor doesn't underdstand what you're saying, then why are you talking to him? And, besides, in this case, since ALL humans spoke the same language previously, if a person saw that his neighbor didn't understand his current language, why couldn't he just communicate to him in the old language, or was it that G-d made all humanity forget the original language?