How to changing Bartik's "Show - Main navigaton" mobile menu message?
You can translate following strings through the core translation system (admin/config/regional/translate):


Show &amp;mdash; @configuration.label
Hide &amp;mdash; @configuration.label


The strings above are defined in block--system-menu-block.html.twig at line 17 and 18.

Furthermore you should translate/rename your Main menu name.