It's saved as a BLOB under users > preferences.
EDIT: For example, if I download the BLOB and open it in a text editor, I get this string for a test account where I set the language to German: (formatting edited for legibility)
{
"notify_discussionRenamed_alert": true,
"notify_postLiked_alert": true,
"notify_discussionLocked_alert": true,
"notify_postMentioned_alert": true,
"notify_postMentioned_email": false,
"notify_userMentioned_alert": true,
"notify_userMentioned_email": false,
"notify_newPost_alert": true,
"notify_newPost_email": true,
"followAfterReply": false,
"discloseOnline": true,
"indexProfile":true,
"locale": "de"
}