Salaam ...
I cannot be certain what the Prophet would have exactly recited in his prayers. I do feel however that his prayer would have been quite dynamic and very spiritually involved. If you understand the Arabic of the 'Tashahhud', you will note that it seems like a prayer that was added later. There is nothing in the Quran to say that the Quran needs to be recited in salat. However, there are many beautiful prayers in the Quran, I think it is a good idea to use them. I also don't see anything in the Quran which prohibits one from reciting the Quran in prayer either. The Quran is silent on this matter so it is left as a choice.
However, the requirement of salat is that you
KNOW what you are saying (hatta ta'lamu ma taquluna - 4:43). Otherwise it is the same as going into your prayer in a state of 'sukara' which is a mind befogged / intoxicated. There is no difference.
So if you do not understand Arabic, it is pointless to pray in Arabic. Did Prophet Jesus pray in Arabic? Or Prophet Moses? Or any of the Prophets? They all prayed in the language that they understood. Of course, it is better if you learn Arabic properly and if you then understand it properly, then to pray in it is fine. However, God understands all languages.
Please see the following articles.
DO WE HAVE TO PRAY IN ARABIC?
http://quransmessage.com/articles/do%20we%20have%20to%20pray%20in%20arabic%20FM3.htmWHY WAS THE QURAN REVEALED IN ARABIC?http://quransmessage.com/articles/why%20in%20Arabic%20FM3.htmI hope this helps.
Your brother ...