Why have a common tongue, that is specifically a common tongue? It is so a D&D convention, like 100 copper, 10 silver, 1 gold exchange rate that never occured in history.
There was never really a "common" on Earth. English came close, as it was the language created when Norman Knights tried to pick up Saxon wenches. This pidgon tongue became a real national language. There is a tribal language in Africa that has no native speakers (all killed off) but is still spoken because it is a tongue that many people knew. (Sorry, don't remember the name... some offshoot of Bantu).
If you are going to have a common tongue, where did it come from? Is it a mismash like English, so everyone speaks it at a limited level using their own language as a basis? Did it belong to a people long dead, "Latin" is that way. What was the language it came from (English came from Norman French/German and Saxon Celtic tongues)? Did some conqueror come down, beat the crap out of everyone and say, "you will now speak this." The English did that in India, and it actually helped the country as more people could actually speak to each other.
In any multilanguage situation, nobody is going to speak the common language with the same fluancy as their native tongue. There will be odd words and concepts that won't translate well. I have found giving each language "a foible" or five, you can give people a feel for the native language people speak.
For example: A russian says Friend or Torvavitch all the time, goes Da and Nyett for yes and no, and has short stucco responses. (Russian is a very direct language). Compare that to a Yiddish speaker, speaking English. The responses will be musical, with lots of hand gestures and "oys" "throat clearings", and odd words thrown in. Think about how the Klingons talk in Star Trek. The tone and the wording shows you they are Klingons. Are these all sterotypical? A bit. But they emphasise the point. You know what that person's native language is. Unless the person has been highly educated they will speak the language oddly (And many Japanese who think they know verbal English at a high fluency come to America or Canada and find out that that is not quite true).
Assign a Foible or five to each langauge. Enforce it in play. Watch it make your languages come alive.