tečan

1. (of language, speech, style) fluent; smooth
   Posle tri godine u Beču govori tečnim nemačkim. — After three years in Vienna he speaks fluent German.
   Knjiga je napisana tečnim i jednostavnim jezikom. — The book is written in a smooth, simple style.
2. (of a substance) liquid
   Kupila sam tečni sapun za ruke. — I bought liquid hand soap.
   Na sobnoj temperaturi voda je u tečnom stanju. — At room temperature, water is in a liquid state.

For language skills the adverb is often used: govori tečno srpski.