I've always imagined that he didn't do NEWT level potions, purely because he would have needed an O at OWL level. In my fic, he dropped it because he couldn't stand Snape. Tonks, however, took it at NEWT because she was a hard worker and bright enough to get the necessary OWL result.
Charlie Weasley was also Quidditch captain as well as a prefect. Percy describes him as an outdoors type. He doesn't strike me as fantastically academic, and Dumbledore didn't choose prefects on their academic ability.
We don't know enough about Charlie, though, so you can go either way.
So I think the biggest change in Charlie post war is that he probably came home more often and was a bit more careful (I always viewed him as an adventure seeking guy who didn't always think about the risks). I think that having Fred die and watching how his mother suffered would just make him a bit more careful when it came to safety and stuff. I think he stayed in England for a while with his siblings, let the wounds heal, but then he returned to Romania. Maybe though, he didn't work with the very vicious dragons anymore. He probably also returned home more often than it seemed he did before the war. Especially since in a couple of years his siblings were going to start having children, get married etc.
I'm working on a fic that includes Charlie, and I had some questions running around in my head about the dragon reserve at which he works. I thought I'd post them here to see if anyone else has thought about this.
- Do mostly Romanians work there? Or is there a strong international presence? Do they speak English or does he need to learn Romanian?
- Why would Charlie give up playing professional Quidditch to work at a dragon reserve in Romania? Is it more prestigious? Or just more interesting to the adventurous Charlie?
- Is the reserve mostly for preservation or is there a lot of research going on there too?
- We just know he works there. What is his job title? What are his duties like?
- Is it a sort of work-your-way-up deal? Like does he start with scooping poop?
I don't think that mostly Romanians work there; I think that the staff is international. I think that this site may be the only dragon preserve in Europe, though I read a charming story by Vittoria, dating from several years ago, about events at a dragon preserve in Tibet.
As for language, I have had nagging questions myself about whether magical persons can use some sort of ongoing translation charm so that when words are spoken to them in a foreign language, they hear the words as if they were in their own native tongue. However, in Goblet of Fire, the French-speaking students from Beauxbatons and the some-sort-of-Germanic-speaking students from Durmstrang seemed to be speaking straight English without the use of any charms; apparently they had learned English. English seems to be a lingua franca in Europe nowadays, but if the magic communities keep themselves isolated from the Muggle communities, that may affect the extent to which English is a lingua franca for them. Still, since Charlie is an intelligent guy and has been in Romania for a long time, I'll bet he does speak some Romanian, maybe a lot of Romanian.
I think that Charlie works at the dragon preserve because it is the career closest to his heart. He was prominent in Quidditch at school because Quidditch was available there and dragon-handling was not, and he apparently goes in for activities that have a strong physical component, but he is not the kind of guy who needs public adulation or a high-profile job in order to maintain his self-esteem. If he is satisfied and fulfilled by what he does, he does not continually look over his shoulder to see if anyone is watching and admiring him. I doubt that dragon-handling is more "prestigious" than being a Quidditch star in the eyes of the general public, though those few people who are knowledgeable enough to appreciate what he is doing will no doubt give him their approbation.
I am sure that a lot of research goes on there too, and that Charlie is involved in it, and not at the lowest or simplest levels. I don't know his job title, but his work involves both theoretical and scientific thinking, and a lot of physical management of the dragons and whatever he is (the whole crew is) studying about them. He is not the type to sit in an office with clean hands and direct everyone else to carry out the actual work.
Although JKR has said in an interview that there were no wizarding universities, and many authors have assumed a type of apprenticeship training after the age of eighteen for entrance into the various professions, other authors have found this idea difficult to swallow and have posited well-described wizarding universities. If Charlie entered his dragon career via the apprenticeship model, then he would have started with the simplest duties and worked his way up, but hopefully with a week-organized apprenticeship curriculum.