What the law looks for
In Czechia, as across the EU, a child seat must carry an approval label to UN regulation R129 (i-Size) or R44/04. The label is usually orange and shows a capital E in a circle.
Seats from the UK
The UK uses the same UN regulations, so seats sold there normally carry an R129 or R44 label. Check the label on your seat.
Seats from the US, Israel and elsewhere
A seat approved only to another standard, such as the US FMVSS 213, has no UN label and is not approved for use in Czechia. Whatever the country, look for the UN label.
The simple option
Rent an approved Cybex i-Size seat at Prague Airport and leave yours at home.