Fix #266.

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
Merge pull request #339 from Tatoeba/language-list-order

Fix #266 (language list collation).