[eluser]Pascal Kriete[/eluser]
It's actually done on purpose. mayl is the language key for the long name.
We have two ways to display the month. It can either say "Dec" or "December". The language keys use the equivalent values. So it's lang("cal_dec") and lang("cal_december"). May is a bit of a problem since it only has 3 letters. In english you could use the same language key, but we can't make that assumption for other languages.
Thanks for the report.