Hello Karel,
just the time span doesn't tell much. In 18th century, what is today the Czech Republic was part of the Austrian Empire, a conglomerate of many different ethnics: German, Austrian (which both speak German), Czechs, Slovaks, Hungarian, Serbs etc.. In my "Haas" case the evidence is clear: all records are held in German language, well back into the 17th century, so she was certainly of German origin.
You might wanna check your ancestor's records, to see what origin they were.