I haven't gotten to the part in-game yet, but I did hear something kinda neat on this. Apparently with the Echo/Hydaelyn's blessing, WoL can hear and understand any other living soul... but only through direct, face-to-face contact. So ex. Yugiri writes a letter in Doman to the WoL. For all she and the WoL know, they were able to converse fluently in Doman for their entire adventure across the nation. But WoL opens the letter and can't read anything whatsoever.

This makes me suspect there might be some very specific mechanic in mind behind how the WoL works (I don't think it's just the Echo at this point) and it allows for ridiculous strength but also very deliberate and particular limits. Idk if it'll happen, but wouldn't be surprised if Shadowbringers examined the limits and nature of the WoL more. Devs seem to be allowing a lot of room for expansion and this is a really fun question that I wouldn't expect them to pass up.