To manage the subtitles of a video, you have two possibilities:

When adding or editing video content in a playlist :

Select the video you want to broadcast and click on "Subtitles":

Un Nouvel onglet s'est ouvert sur votre navigateur :

Click on "Choose a file" and select your file in your explorer.
Select the language of the imported file:

Click on "Import".

The first subtitle language you import is considered the "default" language.

Return to the first tab to finalize adding the video to the playlist:

Click on "Save".

Go to the "Library" tab:

Click on the video you want to subtitle:

Click on "Manage subtitles":

Click on "Choose a file" and select your file in your explorer.
Select the language of the imported file:

Click on "Import".

The first subtitle language you import is considered the "default" language.