Which operating system, which LibreOffice version and how did you install the language pack?
But when I try to open it I get a message
You don’t open a language pack - this makes no sense. You need to set your LibreOffice to make use of the language pack by setting (as being told) Tools -> Options -> Language Settings -> Languages -> Language of -> Option: User Interface
and selecting your language (If you are on macOS then path is:
LibreOffice -> Preferences -> Language Settings -> Languages -> Language of -> Option: User Interface
)
Installing and Using a language pack consists of 2 steps:
- Install the language pack (using your operating system’s method to install software)
- Configure LibreOffice to use it (
Tools -> Options ...
and so on)