Twitter advertisers another way to define the target group of their ads. This is based on the language that users have set in their profile and the language they use in their activity. Therefore, a user can be considered to know two or more languages, if for example they have set their profile language as Greek, but they write tweets in both Greek and English.
This new addition allows advertisers to target users to promoted tweets and promoted accounts based on 20 different languages , which are as follows: Arabic, Chinese, Dutch, English, Finnish, French, Hindi, Indonesian, Italian, Japanese, Korean, Norwegian, Persian, Polish, Portuguese, Russian, Spanish, Swedish, Thai and Turkish. Greek is not currently supported
Advertisers can either use the new language targeting feature on its own to target users who speak the language of their choice, or use the new feature in combination with other features, such as gender targeting, location targeting, etc., to narrow the target group.
Source: socialmedialife.gr

