Scribus is a great and open source software. But, there is no spell checker in released versions of Scribus.
Nobody want to spend time on checking spell by himself, neither do I.
I use a indirect way to spell check Scribus document.
Here is the tutorials:
Step 1 Transfer your Scirbus document into PDF.
Step 2 Download Free PDF to Word Doc Converter and install it. Launch it.
Step 3 Convert your PDF to Word Doc. You can uncheck "Retain shapes and images" to keep filesize small and save time.
Step 4 Use a word processor to check spell and correct your Scribus document. Moreover, use even a grammar software to check spell, grammar, and usages.