From their help:
Click the Tools menu.
Select Options.
Click the Under the Hood tab.
Click Change font and language settings .
Click the Languages tab.
Select the "Check spelling" checkbox if you want Google Chrome to automatically check your spelling.
Select the language you want to use in the "Spell-checker language" drop-down menu.
Click OK to save your settings.
Add a new word to the dictionary
If the spell-checker keeps underlining a word that you often use, right-click the word and select Add to dictionary. At this time, it's not possible to remove a word from the dictionary.
Temporarily turn off the spell-checker
You can also temporarily disable the spell-checker for the field you're typing in. Follow these steps:
Right-click the text field.
Select Spell-checker options.
Deselect "Check the spelling of this field."