In Mac OS X "Tiger" versions 10.4 to 10.4.2, you can't deselect an application's primary system language in the Info window (the window you see after selecting an application in the Finder and choosing File > Get Info). For example, if you have German set as your computer's primary system language, the "German" checkbox in the Languages section of the Info window will be dimmed.
To resolve this issue, update to version 10.4.3 or later. After updating, you'll be able to deselect the primary system language as long as at least one other language box remains checked. When only one language is checked, it will remain dimmed until at least one other language checkbox is selected.
If for any reason you're unable to get 10.4.3 immediately, you can follow the workaround presented below. We're going to assume that German is the primary system language and that you want to use Logic in English in this example, but the same steps apply for other language combinations.
Now your application will use English, even though the primary system language is German.