How to display language switcher block as buttons, instead of simple links?
First choice
If you wants some design related change you can easily achieve using CSS.

Second choice
Enable twig.config: debug: true

See some template file where you can made changes.


  FILE NAME SUGGESTIONS:
  links--language-block.html.twig
  links--language-block.html.twig
  links.html.twig


Changing markup of the language switcher

Trird choice
Alter _language_switch_links_alter.
How do I alter a language item link?