There was a lot of good work done in 2015 among which the setting of llGetLanguage ( ref mantis ).
I am using it in Stable build 0.8.2.1which prove this function to work perfectly well.
But am I the only one having notice this function lack of support for HG visitor.
In my case this function still replies en-us when using same viewer setting, but with a HG traveler coming in the grid instead of being a local user.
(can be validated by visiting 3d.gimisa.ca:9000:gimisa5 region welcome mat) This mat is set for many of the viewer language ( singularity) with greeting sound and chat message, but only work properly for local users.