People write in their native language.
A brief reply, what it means " in their native language".
When the mailinglist ist a multilingual list, list members
can post in their native language. That doesn't mean, someone
translates the emails. This impossible and unrealistic.
When You try to use an online translator (e.g. Google), toss in
Your text and see the result, You'll be delighted, often You'll get
goggly goop. Languages can be very different in structure of sentences
and no translator (program) can handle all languages properly.
Eg.g. You post in English and when someone is able to help, You'll
get a response in English, that's how it works.
Have a nice day