ANSWER:

It’s an innovation to raise the hands and make dua after salah. 

If you make the dua after salah once in a blue moon without raising hands, it’s OK but you must not make it a habit as this is the time for doing the prescribed dhikr and you should make dua in your sujood and in the last tashahud before making Salam.

You can make dua in your Salah, both fard and voluntary, in sujood and in the last tashahud, before making salam and in your own language, providing you don’t know how to make that particular dua in Arabic.

Sometimes after finishing praying with the Salam I raise my hands & make Dua to Allah in my language (since I can’t speak Arabic). I know that dua should be said while still in the prayer but I struggle with memorizing long Arabic Duas & feared of making my prayers invalid if I say it in my language. Is that allowed? I don’t do it routinely & dont do it in congregation.