Depending on the plugin hook, the uid is included in the payload.
How to get locale?
How can I get the user's locale. If possible, can I get it on the client side? If not, what is the user field?
@pitaj on the client side, there is a global variable
config, there are some useful things there
config.userLangis what you asked for.
As for server-side, you'll want
language, I believe, from inside the user set. This is what we use:
user.settings.language || meta.config.defaultLang || 'en_GB'