Blue
Ravenous Bugblatter Beast of Traal
This guy is a veteran. That means he's a survivor. He's seen combat, illness, injury, low supplies, ambushes (from both sides), retreats, city sacking and survived.
This guy is also a campaigner. He knows things. He knows how to dig a latrine. He knows how to cook something of dubious quality and make it edible. He knows that this is just the place where the cavalry is going to charge through and we should get the hell out of here. He knows how to bribe a dog, or a judge. He knows what tool to carry with him and what to leave behind. He knows how to use each of said tools to solve at least 3 different problems.
Lastly he's well traveled. He's been places. He knows the basics in a smattering of languages. He's eaten foreign foods and strange alcohols. He's been on a boat, maybe survived a wreck or two. He knows other cultures, what they want, how they think.
All of this - if it happened during play then it's likely it applies to every character - the wizard just as much as the fighter. So it shouldn't be class based.
If it happens to take then class .... well, it doesn't. A 1st level fighter need not be a first time veteran and campaigner who's well traveled. He might be the village blacksmith's apprentice who's parents were just killed by goblins, or whatever.
If you want it before taking the class, 5e has backgrounds. Now, I think backgrounds should be more impactful, but fitting the above in.