Drupal core language module, language switcher block
You can copy the core/modules/system/templates/links.html.twig into your theme's templates directory, call it links--language-block.html.twig  and in this file adjust the output from the default ul/li structure into a different html structure. 

NOTE: there is already 3 maybe more modules that convert the language switcher to dropdown lang_dropdown, dropdown_language and bootstrap_languages.