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.
A template is quite complex. The libraries are just one of the parts that make up what is translated. ArchiCAD itself could be translated (the menus etc). You have your attributes (layers, fills, surfaces, pen sets, etc etc), graphic overrides, renovation filters and dimension preferences which all likely be in Czech (have Czech names).
As pointed out: your library might have some library parts that are exclusive to the Czech version.