Support geozone segments with non-Latin characters

The `parameterize` method uses the `I18n.transliterate` method, whose
documentation says:

```
I18n.transliterate("Ærøskøbing")
=> "AEroskobing"

I18n.transliterate("日本語")
=> "???"
```

That means we can't use it for dictionaries where characters don't have
a transliteration to the latin alphabet.

So we're changing the code in order to only transliterate characters
with a transliteration to the latin alphabet.

Note the first example ("Česká republika") already worked with the
previous code; the test has been added to make sure accented characters
are handled properly.

lib/user_segments.rb

@@ -84,6 +84,12 @@ class UserSegments
     end
 
     def self.geozones
-      Geozone.order(:name).map { |geozone| [geozone.name.parameterize.underscore, geozone] }.to_h
+      Geozone.order(:name).map do |geozone|
+        [geozone.name.gsub(/./) { |char| character_approximation(char) }.underscore.tr(" ", "_"), geozone]
+      end.to_h
+    end
+
+    def self.character_approximation(char)
+      I18n::Backend::Transliterator::HashTransliterator::DEFAULT_APPROXIMATIONS[char] || char
     end
 end