[biztos]

Anecdata: in 2005 I dated a Czech who called the country "Czech" when speaking English (her third language).

Further tangent: at least part of it has been "die Tschechei" in German since 1918 per the Wiktionary[0] and that's certainly the word people used in West Germany in the 80's, and I believe it's used widely today despite "Tschechien" being official[1].

The Wikipedia article (in German) suggests Tschechei is from Čechy which seems to be Czech for Bohemia. But I am not Czech nor Cseh nor tschechisch nor Čech so I may soon stand corrected. :-)

[0]: https://de.wiktionary.org/wiki/Tschechei [1]: https://de.wikipedia.org/wiki/Tschechien