Fix #266 (language list collation).

If available, use ICU to sort language lists according to the interface
language locale collation rules. One must install the intl PHP extension
to get that working.

From what I can tell, it's doing a good job for:
• German: Dänish goes before Deutsch instead of the opposite
• French: éwé is now at the end of the E’s instead of an the end of the whole list
• Spanish: Árabe is in the middle of the A’s
• Japanese kanjis are ordered differently but I can’t tell the logic; it can’t be as clumsy as PHP’s natcasesort() anyway