I recently needed to install the German Language Pack on Domino 11.0.1, which runs on Windows OS in Czech environment.
After starting the installation, the installation wizard was Portuguese without the option to change the language to German. I thought it was a bad installation package, but the problem was still the same.
HCL Support confirmed to me that he used the same package and everything is OK.
I thought of trying to install and switch to the English Windows environment.
Then I tried to install the Language Pack again and the option to select German appeared and the language pack has been installed OK. Then I switched back to the Czech environment.
Maybe it’s just a problem of the Czech Windows environment, but if you encounter the same problem in your language, this is the solution.