According to the spreadsheet above Chaos is English/French majority and Light is English/German majority. They probably just feel more free to speak their language now. You can't really fault them for that.
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Sure, you can't blame anyone for speaking the language they are most comfortable with, but it feels like a weird move on SE's part to support the division of the community like this. If Chaos gets established as "The French Data Center" and Light as German, it's just going to get worse.
When the servers were all at Japan, since it wasn't devide at all. Everyone spoke english and that was it.
It was way simpler in a way.
Thats how it should be in general. Imo you should always use english if you are playing on international servers with people that you don't know. Sadly people refuse or are not able (which is sad in these times except they are way older than 40) to speak english.