The LANGUAGE response is ambiguous for the corner case where the LANGUAGE
extension
is supported but only the i-default langauge is supported.
Specifically, Section 3.3 states:
A LANGUAGE response with a list containing a single language tag
indicates that the server is now using that language. A LANGUAGE
response with a list containing multiple language tags indicates the
server is communicating a list of available languages to the client,
and no change in the active language has been made.
However, for the corner case the server's list of langauges is just i-default.
Adding a requirements that "IMAP servers that support this extension MUST
support at least one langauge in addition to i-default" would correct this by
avoiding the corner case. (Just an observation, I'm not set on any particular
solution.)