Because it is only logical.

Slovene student

• Slovene is the first language, or, with another term, the mother tongue of most Slovenes.
  
• Slovene is the state language of the Republic of Slovenia.

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Slovene student

• Language at universities (or anywhere else) is not only a means for information exchange. It also has a cultural, identity, symbolic, and emotional dimension. 
  

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Dean of a Slovene faculty

• Most of the students and professors agree that it would make no sense at all if lectures for (mostly) Slovene listeners were in a foreign language (that is English).
  

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