It is not currently possible to set a default spell-check language to be used used for new documents. Once a document has been created, the language can be changed within the document settings but this only affects that specific document.
Whenever I create a new document, it defaults to US English - I presume this is because our account’s data region is set to the US. (We’re a multi-national) Unfortunately this means that whenever I create my own documents, I have to manually set the spell-checker language to UK English each time. It would be nice if the default could be set within the user-settings area.