The tinymce-rails-langs gem adds language packs for TinyMCE (with tinymce-rails).
The gem currently includes all language packs available from https://www.tinymce.com/download/language-packages/, some of which may be incomplete.
Add the tinymce-rails-langs gem to your Gemfile
gem 'tinymce-rails'
gem 'tinymce-rails-langs'
Language files will then be available during development mode and will be copied across when the assets:precompile rake task is run.
See the tinymce-rails project for further integration instructions.