The language the Twitter user selected from the drop-down list on their Settings page.
Note that the value may not be reliable. For example, a user might select French as their mother tongue but compose their Tweets in English. DataSift offers an augmentation called Language which determines the language based on a statistical analysis of 3-letter groups of characters.
The drop-down list shows the full name of the language (English, French, German, and so on) but the twitter.user.lang target contains the two-character code for the language.
Take a look at the list of languages that Twitter supports.
1. Filter for Retweets of Tweets from authors who specify French as their preferred language:
2. Filter for Retweets of Tweets from authors who specify French as their preferred language but Tweet in English: