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.userLang is 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'