Also set $wgContLang in MWITC::setContentLang()

Also set $wgContLang in MWITC::setContentLang()

This assignment got lost in change Ie7a89f6ed7 (commit 47464abb4f) – the
resetServices() now implicit in setMwGlobals() resets the
ContentLanguage service, but does not update the $wgContLang global.
setService( 'ContentLanguage' ) does update it, but that method is
deliberately not used in this branch of setContentLang(), for reasons
that I assume are still relevant even though the comment explaining them
was removed – see change I8c60e37c17 (commit c70c2e4714).

Bug: T231630
Change-Id: I0170cc6bf40a5524523c45e8ffff8f72adc2fc47