Originally Posted by Salzella
The translation point: not an issue. How do all the species communicate in the first place? Drell and Turians sure as sh- do not speak English. In a conversation in ME2 with Thane, Shepard mentions that her 'translator just glitched' when he calls her 'Siha'. This, to me, implies some sort of universal translation device, something that is hardly inconceivable given that it would be a top priority of any set of civilisations looking to form a galactic government. So: the Protheans are extremely intelligent, we know this. It's not a massive stretch to imagine some way (i'm no expert, but w/e) of merging the Prothean language into this thing.
That's how I see it anyway.
You forgot one thing: Prothean is a dead language. There are no known translations and no known speakers of the language (Shepard probably being the exception on account of the Cipher).
And regarding the translation device - a quote from a codex entry:
"Newly discovered or obscure races don't have machine translation available until the linguists have had time to study them."