To prevent losing translation after theme update you need to place translation .po and .mo files into the /wp-content/languages/themes/ folder, also I would recommend you use Loco Translate plugin to theme translation.
Button Sync (Loco Translate plugin http://prntscr.com/glemdq) allows you to synchronize your translation file with updated pot file, after theme update. It means if we made any changes in template files that may affect transaltion or we added new string for the translation and you updated theme you need to synchronize your translation with updated files. Only after synchronization you’ll be able to translate new strings or strings that were changed.
Also, don’t forget to backup all the files before any update.
