This option is only available at FlixGrab application.
First option (for current video):
Copy a video link from your browser to the clipboard;
Paste a link to the application by clicking "Paste a link" button;
Select subtitle language in the dropdown menu;
Click "Download" button.
Second option (for all downloading videos):
Go to "Setting" tab;
Select subtitle language in the dropdown menu;
The application will update the settings in the background.