FieldWorks uses Hunspell for spell checking.

Following are dictionaries for some of the most common analysis1 languages. More languages can be added and if you have dictionaries you would like to share with others, please send them to Flex_DevTeam@Sil.org. To use them in Windows, download the zip file of which ever language you want and extract the contents. Save the extracted *aff and *dic files in the hunspell folders. The location is
C:\%appdata%\hunspell or C:\Users\<userlogin>\AppData\Roaming\hunspell (Windows Vista/7/8/10/11), or
C:\Documents and Settings\<userlogin>\Application Data\Hunspell (XP).

Once the dictionary files have been saved in the appropriate location, the language is available for selection as a spelling dictionary.2

1Spelling dictionaries for vernacular languages are treated differently. They are output from FLEx. See the help topics Spell Checking vernacular words and Vernacular spelling dictionary files for more information.

2To update which spelling dictionary to use, go to Format -> Set up Analysis Writing Systems -> General tab, then under the “Spelling dictionary” label, select the language to use.