I assume other languages have similar things.
The german ä, ü and ö are displayed in the tooltips, but I have another problem:
There is no string for "description (optional)" in Editor.english.php, so I cannot translate it and this text for inserting a link is displayed in english. There are only very simular strings for inserting images in the language files.
Maybe something has to be changed in GenericControls.template.php?
For example this line there is showing an 'en_US'
locale: "', !empty($editor_context['locale']) ? $editor_context['locale'] : 'en_US', '",
I cannot find "locale" in the language files, just for example this string: $txt['lang_locale'] = 'de_DE.utf8';
So maybe 'en_US' has to be replaced with 'de_DE.utf8' in GenericControls.template.php?