There are few issues related to i18n:

1. Currently only strings from the PHP file are available for translation: https://translate.wordpress.org/projects/wp-plugins/insert-special-characters/dev/de/default/
   To fix this the source files should get committed as well so wp i18n make-pot (used on w.org to parse the files) can pick them up.
2. The script has no text domain assigned via wp_set_script_translations().
   This prevents WordPress from loading the correct translation file for the script.
3. Group names and the filter button isn't translatable.
   This needs likely a change to react-character-map since the strings are hardcoded in https://github.com/Dayjo/react-character-map/blob/master/src/component/CharacterMap.js#L175-L185. Would be nice to pass labels via properties.