Matthew Guyan, in the Articulate community, wrote this great piece on 3 different ways to add subtitles (closed captions) in Articulate Stroyline:
From a localization point of view, the best and “correct” way would be the “Use Variable” method. For clients, most of the time, it is imperative to sync the text to the action. For the localization process, exporting and re-importing this text is a must for an easier cheap translation.
As a side note, just keep your lines short at creation, so you don’t have to divide in more chunks your subtitles.