Sorry for the late reply.
Unfortunately, you cannot change spelling default language since it is based on the language of the document. Workspace uses certain templates for every new document and these templates are corresponding to the language of the portal.
You can use customized document templates as a workaround solution in this case. First, you have to create new document templates:
- create new empty files of docx, xlsx and pptx formats and set the language to the desired one;
- save them with next names new.docx, new.xlsx and new.pptx.
Now connect to your Community Server container and get to next folder /var/www/onlyoffice/WebStudio/Products/Files/DocStore/new/
In this folder you will see different catalogs containing templates for every language of the portal. Open the folder with the language that you are using on your portal, for example, English (United States) is en-US (to check it out click on your name > Profile), and replace existing files with recently created ones.
From that point, when user creates new document the Document Server will use these templates for every new document so the language will be set to the document by default.
Note that if your users are using different languages of the portal (they can be set for each user personally) then you will have to replace templates in each folder that is corresponding to the used on the portal language.