Is it okay to make dua inside of salat like while prostrating or in tahsahhud in any language that you speak if you do not know Arabic?
Of course you can speak any language when making du'a in salat. We are recommended to make du'a to Allah frequently, but never is it stated that this must only be in Arabic. Each nation was sent messengers who spoke their language. Other books were sent in, for example the Torah, in Hebrew not Arabic.
It is recommended to learn Arabic in order to study the Quran in its original language, as it is the final message from Allah, the final Book sent to humankind, and has not been changed or adulterated. Meanings can be lost or misconstrued in translation, so it is best if more Muslims learn Arabic to contribute to their knowledge. Also it is best to learn a few basic surahs of the Quran to read in salat, because this is the language in which it came, and the language of Prophet Mohammad.
But this does not mean you are prohibited from calling upon Allah in your own language including in salat. Also you do not have to say your du'a aloud in that case either. But if you so it would seem a not-too-loud approach would be best. (The Quran recommends this.)
And of course, Allah hears and understands all things and all people, regardless of language. And you are ultimately praying and making salat to Allah with no intermediary. No translation therefore required. Allah created people of many nations and languages that we might know each other; it would make no sense to handicap so many Muslims in their salat.