They seem to be sent in the visitor's triggering web cron language.
At least 3 users reported receiving reports in random languages every day.
I cant reproduce this. I created a new user, the report is sent in the users's language. I deleted the user language from user_language table. Then generated the report using japanese browser. Report is sent in english. Maybe this was fixed as a side effect of #2960 ?
Actually I can reproduce. Will work on a fix
(In ) Fixes #2950