utils.l10n: default to en_US when an invalid locale is set
utils.l10n: default to en_US when an invalid locale is set

javiercantero/streamlink,streamlink/streamlink,bastimeyer/streamlink,beardypig/streamlink,melmorabity/streamlink,gravyboat/streamlink,chhe/streamlink,melmorabity/streamlink,streamlink/streamlink,chhe/streamlink,back-to/streamlink,gravyboat/streamlink,beardypig/streamlink,bastimeyer/streamlink,javiercantero/streamlink,mmetak/streamlink,back-to/streamlink,wlerin/streamlink,wlerin/streamlink,mmetak/streamlink