How do I force Outlook to use a given proofing language all the time?

It's because the automatic language detection feature is turned on. To switch it off:

1. Open a new e-mail message.
2. On the Message tab, in the Proofing group, click the arrow under Spelling.
3. Click Set Language Button image.
4. Un-check the Detect language automatically check box.

Inspired by Onxx's comment, I found the solution;

• Tools, Options
• Spelling and AutoCorrection
• Popular (?!)
• Language Settings
• Editing Languages -> Set "Primary editing language"
• Editing Languages -> Remove undesired language from "Enabled editing languages"

I'm using Outlook version 14 packaged with Microsoft Office Professional Plus 2010.

In order to force the proofing language to be my dialect of English, I choose "File > Options > Language", then (if it's not already present, "[Add additional editing languages]" and) specify my language as the default in the "Choose Editing Languages" pane, and remove the other languages. Outlook requires a restart after this change.