L10N USE_EXPAND variable replacing LINGUAS
The L10N variable is replacing LINGUAS as a USE_EXPAND, to avoid a
conceptual clash with the standard gettext LINGUAS behaviour.
L10N controls which extra localization support will be installed.
This is commonly used for downloads of additional language packs.
If you have set LINGUAS in your make.conf, you most likely want to add
its entries also to L10N. Note that while the common two letter language
codes (like "de" or "fr") are identical, more complex entries have a
different syntax because L10N now uses IETF language tags. (For example,
"pt_BR" becomes "pt-BR" and "sr@latin" becomes "sr-Latn".) You can look
up the available codes in profiles/desc/l10n.desc in the gentoo tree.
A detailed description of language tags (aka BCP 47) can be found at:
https://www.w3.org/International/articles/language-tags/