How to add a spelling dictionary in the desktop app
To enable spellcheck and install a dictionary, you have to open the Translation editor. In the bottom right corner of the screen click on Spelling: Choose a dictionary
From the menu select Install
A new dialog window will show up from which you can select the available languages and click install. Currently available dictionaries are for: English, French, Polish, Russian, Spanish and Swedish.
After you've selected the dictionary for a particular language click on Install Selected.
The dictionary for the selected language is now available to choose from the menu.
In case there is no spellcheck dictionary for your language, you import a dictionary file .OXT by clicking on the External Dictionary File button
Finally, select the Dictionary Language(s) from the list and click OK
The Imported dictionary file will be available now to choose on the list.