Turkish has around 10–15 million native speakers in Southeastern Europe and 55–60 million native speakers in Western Asia. Here's what makes it extremely hard to master:

"The distinctive characteristics of Turkish are vowel harmony and extensive agglutination. The basic word order of Turkish is subject–object–verb. Turkish has no noun classes or grammatical gender. Turkish has a strong T–V distinction and usage of honorifics. Turkish uses second-person pronouns that distinguish varying levels of politeness, social distance, age, courtesy or familiarity toward the addressee. The plural second-person pronoun and verb forms are used referring to a single person out of respect." (from Wikipedia).