- Slovak is a member of the Western Slavic language group.
- Bulgarian is a south Slavic language, closely related to Serbo-Croatian and Slovenian and more distantly to Russian.
- Serbo-Croatian belongs to the Slavic branch of the Indo-European language family.
- Macedonian is a South Slavic language in the Indo-European family whose closest relatives are Bulgarian and Serbian.
- Ukrainian is an Indo-European language of the Eastern Slavic group.