Do you use ONLYOFFICE Document Server in integration with certain storage app, e.g. Nextcloud or ownCloud?
Since the language of document is set inside the document itself and not in the properties of editor you can use common templates to create documents with pre-set language. To do so, first, you need to create a new document, change it’s language to desired one, save it and download to your PC. After that navigate to your connector app of ownCloud (Settings > Additional > ONLYOFFICE Section), find the Common templates section and upload your document with set language.
When you create a new file in ownCloud choose the newly added template and the document will be created with the language that you set up for it.