The Collaborative International Dictionary of English v.0.48:

Czechoslovakian \Czechoslovakian\ prop. n.
   a native or inhabitant of Czechoslovakia. Since the
   dissolution of Czechoslovakia into the Czech republic and
   Slovakia in January 1993, this term no longer refers to a
   citizen of any current country. The natives of the former
   Czechoslovakia are now Czechs or Slovaks.

   Syn: Chechoslovak, Czech.
        [WordNet 1.5 +PJC]

The Collaborative International Dictionary of English v.0.48:

Czechoslovakian \Czechoslovakian\ prop. adj.
   Of or pertaining to Czechoslovakia. Since the dissolution of
   Czechoslovakia into the Czech republic and Slovakia in
   January 1993, this term no longer refers to any current
   country.

   Syn: Czechoslovak, Czech.
        [WordNet 1.5 +PJC]

   2. Of, pertaining to, or characteristic of the inhabitants of
      Czechoslovakia or the Czech Republic; as, the Czech
      population.

   Syn: Czechoslovak, Czech.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Czechoslovakian
    adj 1: of or relating to Czechoslovakia or its people or their
           language; "The Czech border"; "Czechoslovak
           nationalists"; "The Czechoslovakian population" [syn:
           Czech, Czechoslovakian]
    n 1: a native or inhabitant of the former republic of
         Czechoslovakia [syn: Czechoslovakian, Czechoslovak,
         Czech]