I am using the fact that there are over 10 languages with no core design on spread nor additional rules.Do we have design insights to that effect, or are we making assumptions based on our subjective points of view?
A party of 4 can't cover all Standard Languages.
And if you count rare languages as 1/2, a party of 4 who all choose not to overlap still has a 35% chance of not knowing what someone says.
And that's before you count languages that are not on the standard or rare list or planes that have additional languages.
So the use of language in D&D even in 5th edition is purely Mother May I.