Translation of the plugin takes place at https://translate.wordpress.org/projects/wp-plugins/simple-history/. When the plugin is installed WordPress downloads language packs from its servers. This means there is no reason to submit language files to the Github Repo of the plugin.
To add a translation of the plugin to your language or update an existing translation, follow the guides at https://make.wordpress.org/polyglots/handbook/translating/first-steps/ to get started.