Fallback to LC_MONETARY when formatting currency

[afh]

POSIX defines the LC_MONETARY category to be used for monetary formatting, hence babel could be more standards-compliant in this regard.

[afh]

Friendly ping to @akx and @oprypin to get some eyes on this. Not sure if you are the right folks to ping, yet looking at other PRs it seems you would at least know who else to get involved 🙂

[akx]

This is technically okay, but we really should fall back to LC_NUMERIC if LC_MONETARY isn't set so as to match the old behavior for people who don't have LC_MONETARY set; right now default_locale would then fall back to LC_ALL, LANG, etc.

You'd need to change default_locale() to accept multiple "primary options" before those fallbacks.

Furthermore, previously, LC_MONETARY was ignored altogether, but now it will be used if set for money formatting, which could be a surprising change for people who have it set. This change will need to be thoroughly documented in the changelog.

[akx]

This documentation doesn't seem to match the new behavior; it's not an "or" if the envvar is set. This should probably say

Suggested change

	* ``LC_NUMERIC``,
	* ``LC_NUMERIC`` or ``LC_MONETARY`` for currency related functions,
	* ``LC_MONETARY`` for currency related functions,
	* ``LC_NUMERIC`` for other numbers,

or something along those lines.

[afh]

Thanks for the review and feedback, @akx, much appreciated. I'll look into addressing your comments and updating this PR accordingly. Not sure when I can get to it, but I'll try to make time soon.