By default, httrack stays on the same address.

Because you started on praha6.zeleni.cz, httrack will ignore
strana.zeleni.cz.

If you want to grab stuff from both subdomains, try the option
--stay-on-same-domain.

It should let httrack visit everything in the zeleni.cz domain.

Alternatively, you could try this filter: +*.zeleni.cz/*

The filter should have the same effect as the other option.