Kajkavian
Kajkavian (Croatian: Kajkavski) is one of three vernaculars of Croatian language, spoken in Northern Croatia, including capital of Zagreb. It was named after "kaj" pronoun which means "what".
First texts and mentioning of Kajkavian vernacular originate in 12th century, though first published work date back to 16th century. Kajkavian has been used as literature language. Most of Croatian grammar written before the standardization is written in Kajkavian language and it was considered "Croatian language" in foreign circles as Northern part of Croatia was called "Kingdom of Croatia". Kajkavian is less mutually intelligible than other two vernaculars.
Due to it's rich history and own vocabulary and grammar it is considered as an language itself.
Around 1.3 million native speakers. It has 15 dialects.
Examples of Kajkavian:

Kaj se dela? (stand Cro: Što se radi?) - What are you doing? What's up?
Ne bu išlo! (stand Cro: Neće ići!) - It's not going to work! / No way
Deni čučeka v šporet. (standCro: Stavi pile u pećnicu.) / Put the chicken into the oven.
Koj si rekel? (stand Cro: Što si rekao?) - What did you say?