FEATURE: Adds transliteration of German umlauts in slugs

- Moves the already existing transliteration rules into `transliterations.en.yml` (there's no need to translate this for every language). The same goes for the stringex configuration.
- Doesn't calculate the default slug for *zh_CN* and *ja* anymore. It hasn't been used anyway since stringex is used instead.
- Removes a wrong comment from the Russion transliteration file (I hate wrong comments)

lib/slug.rb

@@ -4,8 +4,6 @@
 module Slug
 
   def self.for(string)
-    slug = string.gsub("'", "").parameterize
-    slug.gsub!("_", "-")
     # TODO review if ja should use this
     # ko asked for it to be removed
     if ['zh_CN', 'ja'].include?(SiteSetting.default_locale)
@@ -13,6 +11,9 @@ module Slug
         require 'stringex_lite'
       end
       slug = string.to_url
+    else
+      slug = string.gsub("'", "").parameterize
+      slug.gsub!("_", "-")
     end
     slug =~ /[^\d]/ ? slug : '' # Reject slugs that only contain numbers, because they would be indistinguishable from id's.
   end