You can use the menu: Tools -> Spell Checker -> Spell Check (F7)
It will underline unidentified words.
You can also right-click any word in your text, Text Edit Plus will suggest other words at the top of the popup menu.
Text Edit Plus supports DIC and AFF dictionary files and comes with preinstalled English and German dictionaries. The same dictionary files are also supported by OpenOffice and LibreOffice.
You can find and download dictionaries for a wide variety of languages using the links below:
Copy .dic and .aff files into Dictionaries folder inside Text Edit Plus directory.
The default dictionary folder:
You can use the menu: Tools -> Spell Checker -> Options
Using the options, you can set default spell checker language, explore installed dictionaries, update dictionary list, or add new dictionary files.