i18nUtils: accept 3-letter locale slugs

[deBhal]

This PR fixes #3175, by updating the regex filter in i18nUtils.getLanguage() to accept locales with three-letter slugs.

If you run config('languages').forEach( (lang) => i18nUtils.getLanguage( lang.langSlug ) ? null : console.log(lang.name) ) (e.g. by adding it to the bottom of client/lib/i18n-utils/utils.js and calling make test) you see a list of languages that occur in the config but return null from getLanguage():

als - Alemannisch
arc - ܕܥܒܪܸܝܛ
ast - Asturianu
csb - Kaszëbsczi
fil - Filipino
fur - Furlan
ilo - Ilokano
mwl - Mirandés
nah - Nahuatl
nap - Nnapulitano
nds - Plattdüütsch
non - Norrǿna
rup - Armãneashce
tir - ትግርኛ
udm - Удмурт
vec - Vèneto
xal - Хальмг

These languages all have 3 letter slugs, so I've updated the filter in i18n-utils to allow them and added a test for one of them.

[yoavf]

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