Shouldn't the Voidborn -
- be speaking some other incomprehensible language?
Why do
and
speak the same language as us when they're from a completely different world?
as well, seeing as how he's some primal creature of the earth that spent his life in, more or less, isolation from the rest of the world.
And
is mute yet still has a voice.
Adding a new, fictional language for Runterra would add some depth; however, would the work really be worth the cost? Creating an entirely new language is difficult to say the least, and if Riot does (instead of just copying from another game like you seem to want to do), how many people do you realistically think are actually going to not only take the time to understand it, but like it.
Plus, as I touched upon, if they make a unique language for one particular character, they obligate themselves to making up languages for others, which is probably far more work then needed/wanted.
The reason I think we can understand all the Champions is, because when us Summoners do our mindmelding thing with them, we become synced as one. Thus, no matter what "language" they're speaking, it's mostly translated. Even if it's not (ancient Ionian - and even then it's just one word or something), that language is still based on an actual real-life source significantly cutting down the development-labor of having to construct an entire way of communication from the ground up.
Finally, you assume dragons in Runeterra are the same as dragons in Skyrim in the respect that they have their own language. We honestly don't know a whole lot about the dragons of this game, and for all we know they speak the same tongue as "everybody else". :P