There is no methods for that. You just open the same project in another language version.
ArchiCAD brain is language-independant. The language is only shown on user interface commands.
I guess you will need to use your Czech library, that originally was used for the project. If Czech library is identical to International - then, probably, library replacement can work (not sure about this - needs to be checked). So, if you will have Czech library elements in ArchiCAD INT - they will still have Czech language, because library parts interface is programmed in GDL script and is totally independent from ArchiCAD version.