I have installed the 2009 R2 client, server, middle tier and additional Danish language pack as described in the documentation.
So the NAV installation has Danish (1030) and US-english (1033) languages installed.
When I change language from inside the RTC it works perfectly.
However, when I change the language from the command line using the “-language:1030” (which is Danish) parameter it selects US-english.
I have tried to change the language code parameter to a language that is not installed, for example -language:1040, then I get an error message saying that the Italian language is not installed.