Seekbar Sync Issues in Storyline 360 After Localization – English Audio Longer Than Spanish

If you need other interactions to sync to the soundtrack, you may need to use language layers. But, presuming it is only the end you're trying to resolve, then I'd just add a trigger: When media completes (Audio 1 (EN)) then jump to cue point. And set the cue point to the last moments of the slide. For the long audio, the progress will be smooth. For the shorter audio, it will jump suddenly from 80% complete to 100%, but at least it will appear that it did ultimately complete.

You could also make entirely separate slides and adapt your menu to only show 1 of them.

I've never dealt with multiple languages to the degree you are, only Canadian vs American spelling and localized content. I found a combination of variables to handle spelling and small changes, plus layers to handle larger differences worked best. I used javascript to tweak the menu names to match.