Actually... it's generally easier to use the Language tool on the status
bar. Right-click the status bar (bottom of the Word window), and if Language
is checked, you're good to go... if not, click to check it.
Then, when you want to implement Suzanne's suggestion, select the text,
click your language on the status bar, and click Do not check... in the
Language dialog box.
If you need this a lot, a good long term strategy is to create a new No
Proofing character style that has "Do not check..." as part of its style
definition (making sure that you save this new style into Normal.dotm so
it's available thereafter when creating new documents). Then, anytime you
want to inoculate text against proofing, select the text and apply that
style.