Calendar months not displayed in actual language.
You can test it at my moodle test site http://rezeau.info/moodle (no need to login) by switching to various languages.
Calendar months correctly displayed for : English, Deutsch, French
Incorrectly displayed for Español, Português, Serbian language (both Latin and Cyrillic).
This looks like an old bug I reported back in 2004!
It may be related to recently reported bug