• use correct env vars LANG and LANGUAGE
• as an additional safety-net explicitly set QLocale default to english

(the latter kinda overrides the former, but better save than sorry since it
has no adverse effects to do both)

this should make tests pass regardless of potentially weird env states some
CIs may have

Fixes bug introduced in D10446

test passes on kubuntu ci. my theory is that QLocale picked the default before the env was set up and therefore used C instead of en thus bypassing the en=>en_US coercion