Smartcat has a failsafe process where segments are stored in the browser cache so the translator can keep working for a while but it's for intermittent outages, it's not meant to be used to work offline. So it should cover for a short period of time (maybe 15-30 minutes depending on the cache size) and then the user would need to reconnect. It's really an emergency system for users with unreliable internet connections, not something to rely on for work.
If users need to work offline, they are better off download XLIFF files that can be processed using many different applications.
