Seems Outlook only has a spelling dictionary dependent on other programs from Office suite, such as Word. Try these.
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from windows help file;
To check the spelling in messages
Outlook Express uses the spelling checker provided with the following Microsoft Office programs: Microsoft Word, Microsoft Excel, and Microsoft PowerPoint. If you do not have one of these programs installed, the Spelling command is not available.
In the New Message window, click Spelling on the toolbar, or on the Tools menu, click Spelling.
Note
To customize the way your spelling checker works, on the Tools menu, click Options, and then select the Spelling tab.
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Visit the link, a lot more there. Although for Office 2007, it would apply to earlier versions and programs within Office. I'm not sure which one affects the setting for Outlook.
http://office.microsoft.com/en-us/outlook/HA102488711033.aspx
Every language version of the 2007 Microsoft Office system includes proofing tools, such as dictionaries and grammar rules, for more than one language. With the appropriate proofing tool languages available and installed, you can change the dictionary language so that you can check the spelling of a word or a document written in a different language.
To see which languages are included with your version of 2007 Office release, see The 2007 Microsoft Office suites localized versions. If your version of the 2007 Office release does not include the language that you want to use you may need to get a language pack. For more information, see Determine if you need a language pack.
To change the default language and dictionary to a specific language for all your 2007 Office release programs, see Change the default language for Office programs. To use a particular language location, such as English (United Kingdom) or English (Australia), see Set the display and editing languages.
Outlook
Set the dictionary language
Important If you have Outlook Express 6.0 and installed Microsoft Office 2007 but French is the only available spell check language, you need to use a third-party spell checker. For more information, see Spell checking issues with Outlook Express 6.0.
1. In a new e-mail message, select the text that you want to check.
Tip You can specify the dictionary language for a new message before typing any text. Just position your cursor in the message body of a new message and complete steps 2 and 3, type your message, and then check the spelling as described in the Check spelling and grammar section for Outlook.
2. On the Message tab, in the Proofing group, click the arrow under Spelling, and then click Set Language Button image.
3. In the Language dialog box, verify that the correct language is associated with the text. If it is not, select the language that you want to use for the dictionary and other proofing tools. If you don't see the language that you want, scroll to the end of the list.
Note If the language that you want is preceded by the Spelling & Grammar iconButton image , it means that the proofing tools, such as spelling checker, are installed for that language. If the language that you want is not preceded by the Spelling & Grammar iconButton image , the dictionary for that language is not available and you cannot check the spelling and grammar for that language.