This issue was previously reported by mantis bug
issues.asterisk.org/view.php?id=13975
Essentially the problem is that when a user calls into a meetme conference using a different language from the existing conference participants, the existing participants hear the “has joined” message in the language of the joiner, rather than their own language.
The original bug was reported against Asterisk 1.4 and I am now running Asterisk 1.8.11.0.
Is this still regarded as too tough to fix ?