How did English come to use a variation of the Polish spelling for Czechoslovakia?

Solution 1:

There is a possible Polish (or Latin) connection, but this was made by the Czechs themselves:

OED:

Origin: Of multiple origins. Partly a borrowing from German. Partly a borrowing from Czech. Etymons: German Tscheche; Czech Čech.

NB Č = Cz = [Ch]

Etymology: < (i) German Tscheche (17th cent. or earlier), and (ii) its etymon Czech Čech, self-designation (14th cent. in Old Czech; plural Češi, (inanimate) Čechy); further etymology unknown.

It is uncertain whether the following earlier instance should be interpreted as showing the post-classical Latin or English word:

1610 P. Holland tr. W. Camden Brit. i. 23 They that nominated themselves after their owne language Teutsch, Numidians and Hellenes, by the Romanes were named Germans, Mauri and Grecians. Even so in these daies..they which in their owne Idiome are called Muselmans, Magier, Czecchi and Besermans [L. Muselmanni, Magier, Czechi, Besermanni], are by all nations in Europe named Turkes, Hungarians, Bohemians, and Tartarians.

With use as adjective compare an isolated earlier use as adjective of Chechi < either Czech Čechy (plural noun; also used as a name of the country: see Czechian n.) or perhaps post-classical Latin Czechi (plural noun; see below):

1759 Mod. Part Universal Hist. XII. xv. vii. 127 Being joined by the Chechi (or Bohemian), Pole, Latin, and Bulgarian, Valak Ogli's auxiliaries, he leads them into the Musulmȃn dominions.

With the β. forms compare Old Czech Czech, the usual form in the 15th cent. This was later replaced by Čech in Czech orthography, but the digraph cz was often retained in post-classical Latin Czechi (alongside Zechi; both plural, second half of the 16th cent. or earlier), perhaps partly under the influence of Polish Czech (plural Czesi).