You can add captions (e.g. subtitles) to your Twitter videos. Twitter supports .SRT subtitle files on web, iOS, and Android. 

Step 1. Open Twitter Media Studio Library

Visit your Twitter Media Studio Library. If you have not already, upload your media to Twitter.

Step 2. Select Your Video

Once you have your video uploaded, select your video from the library. To narrow your search results, use the pulldown menu in the top left by the search bar and select "videos." Choose the video you want to add captions to.

Step 3. Select Subtitles

From the Media Details dialog box, select the Subtitles tab.

Step 4. Choose a Language and Upload the .SRT file

Choose the language of your subtitle from the list, and upload the .SRT file.