Tested with the locale sv_SE.

The weather applet seems to always default to using non-metric units.
That's most likely not very clever, since almost all parts of the world
(with the US as the most notable exception) does use metric units.

The locale information (to be precise LC_MEASUREMENT) includes information
about which unit system to use (1=metric, 2=US). It's probably a god idea
to make use of that. If the locale information cannot be found or the
locale is unset, using metric as the fallback is probably a good idea.

In short:
1) Use locale info (en_US would return 2, all other locales will probably
return 1)
2) If the above fails, use metric