This method partially implements the lookup matching scheme as described
in RFC 4647, section 3.4 and thus does not strictly conform to the
specification.
Differences to specification:
- No support for wildcards in the to-be-matched locales.
- No support for locales with private subtags.
- No support for a default return value.
- Passed locales are required to be in canonical form (i.e. 'ja_JP').