Add i18n support for link/image picker

Implement internationalisation for TinyMCE hippopicker plugin. 

CKEditor equivalent: /community/cms/richtext/ckeditor/plugins/src/main/resources/ckeditor/plugins/hippopicker/lang/en.js and siblings, this needs a rewrite for tinymce:

CKEDITOR.plugins.setLang( 'hippopicker', 'en', {
  imageTooltip: 'Image',
  internalLinkTooltip: 'Internal link'
});

TinyMCE supports (at least):

1. a config attribute named content_langs: https://www.tiny.cloud/docs/tinymce/latest/content-localization/ which is set on init.
2. setting it before init: https://www.tiny.cloud/docs/tinymce/latest/apis/tinymce.root/#addI18n