Arthur has a french accent. Ususally it isn't terribly heavy but you can hear it. He's fluet in japanese and normally has no struggles with speaking japanese very fast. I mean, he probably had to talk very fast in japanese, being a spy and working in the laboratory and everything. But. Whenever he gets really annoyed, angry or really emotional he starts ranting or starts to ramble. When this happens, his french accent gets thicker which makes him even more annyoyed. Eventually it reaches a point where one can barley understand him and he starts to mix french and japansese words in his sentences.
Usually he is careful that this never happens when he's out but at home this happens quite often. Usually it starts with him being annyoyed bc of smth and it ends up with him ranting more in french than in japanese aswell as him walking back and forth across the room while making hectic handgestures.
Verlaine is either sitting there, listening to his husband partner, looking at him, being completly smitten and smiling like an idiot or he's trying to calm Arthur down but can barley do so because he's trying so hard not to laugh (He fails all the time).
In conclusion: Rimlaine are soft french husbands and Verlaine adores these rants of Arthur (As long as they aren't about something serious)