Slovenian is a South Slavic language with over two million speakers living mainly in the country of Slovenia. It is also actually spoken in many other countries across Europe. Slovenian has many similarities to Croatian and Serbian. It can be understood by those speaking Kajkavian Croatian dialects.
Slovenian has a standard version that came about from more centered dialects in the 1700s but also has several regional varieties. They may use slightly different words.
Regional varieties have lost their uniqueness slowly, most likely because of more accessibility to the language. However, some places still retain their individuality, such as Udine in Italy. Other standardized speakers would find this variety challenging to understand.