You turn on the spell checker in Spotler by selecting a text element. Then a menu will appear at the top. At the ABC-icon, select the language of the spell checker.
Spotler has an automatic spell checker based on the language settings in your browser. When you see all the text underlined in red, the language of the spell check in your browser is set to English.
You can adjust this via the (advanced) settings of your browser. In the case of Chrome, it looks like this: