Language-selector plugin, change selection

@iclou, You could try the following:

• If not already, please create an inherited theme.
  Let's assume you called it 'mytheme'
• Copy file /user/plugins/language-selector/templates/partials/language-selector.html.twig' into folder '/user/themes/mytheme/templates/partials
• In the copied file make two changes:
  • Replace line 7 with: {{ language_selector.current | upper }}
  • Replace line 41 with: {{ language | upper }}