Front uses the spellchecker built in to Chrome. The below instructions explain how to update your spell check settings.
Instructions for Mac
Go to System Preferences.
Then select Text.
In the Spelling dropdown, choose your language.
Instructions for Windows
Highlight a word in your composed email.
Right-click the highlighted word.
Hover over Languages to expand the list of supported languages.
Choose a language.
Note: On some Windows machines, you may need to add a language to your dictionary. To do this, go into your computer Settings and select Region & Languages.
If you have a language selected, spell check will underline the misspelled word, but not change it automatically.