Cover the regression hinted at in review: an already /pl/-prefixed link
must not be prefixed a second time now that pl is part of the language
pattern.
Polish (pl) was missing from the hand-maintained language regexes, so
getLanguageFromUrl() returned en for /pl/... pages and the UI fell back
to English after the initial server-rendered Polish content.
Derive the pattern from supportedLanguages so new locales cannot be
missed again, and cover Polish in getLanguageFromUrl(), changeLanguage()
and rewriteLinks() with regression tests.
Refs #972
Athanasios Panos