Originally Posted by Red Hessian
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."
Huh, didn't even know there was a codex entry. Durrrr. But weren't there more Prothean beacons than just the one on Eden Prime? It's just supposition, but a language isn't dead until there are no speakers of it left. And what about Liara, say? I'd be surprised if she was an expert on Protheans and couldn't speak a word of the lingo. Not saying it's be easy, but it is plausible.