Plugins: Serbian Alphabet
Transliteration, Cyrillic and Latin Alphabet, Serbian Localisation
Finally, the right plugin: Transliterator
Transliteration
Cyrillic To Latin And Latin To Cyrillic Javascript Library
sjinks/wp-plugin-cyrtolat: WordPress plugin to transliterate cyrillic slugs into latin opendatakosovo/cyrillic-transliteration: Transliterate Cyrillic script to Latin script and vice versa.
Sanitize Permalinks
Fixing German Umlauts in Permalinks
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de_DE | bueltge/de-DE
is also German permalink sanitizing plugin -
toscho/Germanix-WordPress-Plugin
is fixing both slugs and filenames
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