I want to disable the default english language. It's ok if it will stay installed in the system, f.e. for IPS support/bug fixes... but I want to deactivate it for all members. They may NOT USE IT. How can I achieve this?

All languages should have a permissions setting, who can see/use the language.

Thanks.

To disable the default English language, you have to set some other language as the default, then the green "Locked" icon for English will change to "Enabled", and clicking it will disable English. (We in support can re-enable it if necessary for testing.)

@Mark H Thanks, that is what it is supposed to do and makes sense.

BUT: I already tried to do this. As soon as I set another (new, foreign) language translation as "default for all groups", the "locked" icon for the english language persists... I can't disable it.

In that case, please file a support ticket. We'll need to look at your install.

I just double-checked on a test install and it works as I described.