Hi @AlexB,
If your customer gives you the video and subtitle files (srt format), I’d recommend you to “burn” the subtitles in the video yourself. That’ll be the easiest / cheapest (free) way to handle subtitles in IF.
Here’s a fast & free way of doing it: How to Add Caption Files to Videos in Handbrake (Subtitles and Captions) | Rev (use “burned-in” option)