add option to choose installed translations using LINGUAS

[cjmayo]

Saves a lot of code in downstream packaging with selected translations only e.g.:
gentoo/gentoo@53c8431#diff-13ffa73c7e68e1ac2f92e7aa89c68cc2R50

[cjmayo]

Maybe L10N isn't the best choice of name for the variable as the language names are in "language_country" form - with the country in lower case. And if they really are just translations and not conversion of units etc.

Any particular reason for the lower case country suffixes? (FApplication::findTranslator)

Selfishly BCP47 format with hyphens and upper case region subtag would be easier to handle. But I guess changing it could break a user's saved configuration.

[cjmayo]

I think LINGUAS Is better, typically uses underscores instead of hyphens.

Upper case country codes would be great but easily converted with ${LINGUAS,,}