Advertisers can now target Promoted Tweets and Promoted Accounts in languages that their audience understands, while users are eligible to see highly relevant ads in their language. Along with language-specific analytics, this targeting is available to all advertisers globally and through our Ads API.Twitter Introduces Language Targeting In Accounts and Tweets [blog.twitter.com]

We use a number of different signals to determine a user’s language, including the language selected in a user’s profile settings and the languages that correspond to a user’s activity on Twitter. A user can be targetable by multiple languages if we infer that a user is multilingual.