In apps like TextEdit, Mail, Safari, etc. (but mainly Mail), I like to use the Multilingual option in the spell check dialog (Edit » Spelling » Show Spelling & Grammar) because I regularly write in English, French and Spanish. However, I really have little everyday use for the other languages, and they pose the problem that a word that may be misspelled in English, French or Spanish may actually exist in another language that is covered by the Apple spell checker.
So I removed all those other languages from the spell checker, and it works exactly as I'd hoped (even for grammar). Read on to see what I did.
First I created a folder called AppleSpellTest inside my folder zzz_Disabled_Stuff that is in my home folder. Then I made a backup of the files I was going to modify by doing this:
$ sudo su
$ mv /System/Library/Services/AppleSpell.service/Contents/Resources/Danish.lproj /Users/*Your_Home*/zzz_Disabled_Stuff/AppleSpellTest/DisabledLanguages
$ mv /System/Library/Services/AppleSpell.service/Contents/Resources/Dutch.lproj /Users/*Your_Home*/zzz_Disabled_Stuff/AppleSpellTest/DisabledLanguages
$ mv /System/Library/Services/AppleSpell.service/Contents/Resources/en_AU.lproj /Users/*Your_Home*/zzz_Disabled_Stuff/AppleSpellTest/DisabledLanguages
$ mv /System/Library/Services/AppleSpell.service/Contents/Resources/en_CA.lproj /Users/*Your_Home*/zzz_Disabled_Stuff/AppleSpellTest/DisabledLanguages
$ mv /System/Library/Services/AppleSpell.service/Contents/Resources/en_GB.lproj /Users/*Your_Home*/zzz_Disabled_Stuff/AppleSpellTest/DisabledLanguages
$ mv /System/Library/Services/AppleSpell.service/Contents/Resources/German.lproj /Users/*Your_Home*/zzz_Disabled_Stuff/AppleSpellTest/DisabledLanguages
$ mv /System/Library/Services/AppleSpell.service/Contents/Resources/Italian.lproj /Users/*Your_Home*/zzz_Disabled_Stuff/AppleSpellTest/DisabledLanguages
$ mv /System/Library/Services/AppleSpell.service/Contents/Resources/Portuguese.lproj /Users/*Your_Home*/zzz_Disabled_Stuff/AppleSpellTest/DisabledLanguages
$ mv /System/Library/Services/AppleSpell.service/Contents/Resources/Russian.lproj /Users/*Your_Home*/zzz_Disabled_Stuff/AppleSpellTest/DisabledLanguages
$ mv /System/Library/Services/AppleSpell.service/Contents/Resources/Swedish.lproj /Users/*Your_Home*/zzz_Disabled_Stuff/AppleSpellTest/DisabledLanguages
$ exit <array>
<dict>
<key>NSExecutable</key>
<string>AppleSpell</string>
<key>NSLanguages</key>
<array>
<string>en</string>
<string>fr</string>
<string>es</string>
<string>Multilingual</string>
</array>
<key>NSPortName</key>
<string>AppleSpell</string>
<key>NSSpellChecker</key>
<string>Apple</string>
</dict>
</array>
Save, restart the Mac, and you're done -- and it really works!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090512175846504