For the Thai locale, it would be desirable to change the default calendar to be
the Buddhist calendar.  But this creates a problem with interpretation of
two-digit years.  If a user does not go and manually change the option in the
Tools dialog, then a year of 48 (which should be interprted 2548 BE) will be
interpreted as 1948 BE (ie 1405 AD, some time in the Middle Ages).  I guess the
language pack could change the option, but even this wouldn't be write: there's
no guarantee that somebody using the Thai locale will have installed the Thai
language pack or vice-versa.

It seems to me that the right solution is that the TwoDigitYear configuration
property should not be a single year, but should rather be a list of (calendar,
year) pairs: eg (Gregorian, 1930), (Buddhist, 2500) etc.

This current UI could be left as is: it would show the TwoDigitYear for the
default calendar of the current locale.