Hi there,
What you need to do, is go into the System Preferences pane and select "Language & Text", as in the screenie below.
Then, when you have that window open, you would need to select the tab marked "Language" and select your language there (It might also be a wise idea to click "Edit List" and remove any un-needed languages. See below for another screenie.
What I'd also suggest you do, is go to the "Input Sources" tab and make sure that your language is ticked and again, remove any un-needed entries.
See below for another screenie.
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