Colmar canonically has a French accent (it has everything to do with the fact he's named after a place in France anyway).
Changsha has a speech impediment in which he "tawks wike dish", meaning he can't pronounce r, l, or s. Obviously this will cause a great deal of problems in communication because nobody takes him seriously, let alone his family or even his close friends.
Halifax... hm... he has a bit of an accent but I can't tell what exactly it is...
Tashkent has had a stuttering problem since he was very young after burning his mouth when learning to breathe fire.
Texas, considering which state he's named after, has a Southern Accent.
Toulouse (at least he insists) has a sort of impediment where he streeeeeettttttchhhhessss theeee sssssoooooounnnnnndddsss of the wooooooorrrrrrdddssss lliiiiiiiikkkeeee thiiisssssssss.
Chongqing often mumbles or slurs his words because of an impediment he cannot control.
Shanshan also has a stutter but it sounds more like he's having a seizure while talking, making it hard to understand what he's saying.