I'm German, on a German server and WvW was the last place where I could chat in German.
Apart from the fact that some players aren't well in foreign languages (see the complains on the German forum - you won't find those ppl here) there are also many that may have no problem with English in general but prefer their native language.
You can't relate?
I've no prob with English in chat.
I have a prob with English in voice. Listening, and speaking is even worse.
But WvW ist also the place in which voice chat brings a great advantage when in a squad.
When my server was linked with non-DE servers, I used voice chat only for listening, never said a word, used the text chat if I had to inform my squad of something (with delay that way ofc), and I hoped that I understood everything well enough to follow orders and suggestion.
Didn't always work. But worked well enough to get the stuff done.
My game itself is in German too, so I often have to think what the commander, or others, mean. It's not always communicated in direct translation but whatever is common to name it on that server.
Some have a site with explanations linked in their TS or Discord. But seriously who reads a FAQ before playing?
I didn't play much when linked to non-DE servers. I played much more when linked to German servers. Because it is just cool and fun and natural to chat with others in your native language, even more so when they're from your country. (German speaking players include players from Austria and parts of Switzerland.)
Whatever, I think it's understandable if people prefer their native languages. I already was pissed about the megaserversystem in PvE. I chose a German server, when creating my account, for a reason.
I want to keep the system like it is now if any update only means disimprovements to me.