every custom variable will be saved global* with leave slide or open slide layer** - on resume all custom variables are restored
if you don't reset the four variables - it can happen, that multiple languages are defined at the same time
iif learner cannot open the language slide a second time and you disable resume on your course, the four trigger are not absolutely necessary - but it is best practice to do it anyway
* to LMS (published to LMS) or in the browser to Local Storage (publish to web)
(on review 360 the save/resume function is disabled !!!)
** this is new with storyline 3.69 (no infos in the release notes !!!)