Further to a lengthy discussion on the French-speaking forum, I have created bug report MDL-80606 for which you might like to vote (and comment).
H5P
Switching languages in H5P Moodle core throws PHP warning
གྲོས་བསྟུན་འདི་བསྡམས་ཡོདཔ་ལས་ ལན་གསལ་བརྐྱབ་མི་ཚུགས་