Even with the Mars Archives giving us a comparable technology, we simply didn't have the time to actually build much of it. The Prothean technology was discovered in 2148, the First Contact War started 9 years later.
Unless the Protheans left information about the other species in their archives (which has never been mentioned to my knowledge), then I don't know why Humanity was motivated to build a massive fleet for their defense.
There's a short scene in the Citadel Archives, showing turian soldiers interrogating a human prisoner at Shanxi, commenting that his rifle was primitive. What I took from the scene is that the universal translator must be the most fantastic piece of tech in the known universe and everyone, even poor slobs in the slums of Omega, can afford one.
Even better... despite the very short time Humanity didn't just build a massive fleet, they had so much spare material that Cerberus managed to secretly build a couple cruisers for themselves without anyone noticing.
It's a miracle that the other species can hold their own against the industrial power of the humans. Without the Reaper invasion, humans would have taken over the entire galaxy within a few centuries. 
Or maybe the writers were more enthusiastic than logical.
Oh, the humans had no idea what alien species were out there. When the First Contact War started, they thought they might be fighting Protheans, since that was the only race they knew about.
And yes, everyone in the galaxy speaks the same language. All that remains now is to see if that extends to Andromeda, or if we need babelfish 
How many cruisers did Cerberus have?
Liara destroyed one by ramming it with her Shadow Broker ship
Aria stole another one
One gets shot down over Tuchanka
At least one other was guarding Omega (almost certainly more)
One attacked Grissom Academy
Those are just the ones I can think of off the top of my head
And there were still enough forces to require Hackett to mobilize a considerable force to take Kronos Station.
It's the Star Forge, I'm telling you. TIM is a Sith Lord and used the Star Forge to build himself a fleet!