To base it off of what you(Vlad) took Valg and Yanoreth to be saying:

Valg is saying that if people would ICly know the same language, they should all understand each other. (this is a good reason to not use your own invented languages, or multiple forms of fantasy Elvish, etc, etc, etc)

Yanoreth is saying that not everyone of the same race would automatically know the same language. (a half elf is the most obvious example of this, as they'd typically learn the language of one parent or the other)

Therefore, while not everyone knows the same languages, everyone who would know the appropriate racial tongue ought to be able to understand it. Without using something like the Lyceum texts as a basis, there is no suitable common ground. So, if you want to speak to someone in a language both of you would know IC, roleplay the discussion. Anyone present who ought not ICly know the language should get the hint and roleplay accordingly.

So, in truth, they're saying the same thing from different angles, just like they each already personally said. By the way, those notes in parentheses are entirely my own examples, just so nobody thinks I'm putting words in anyone's mouths.