The new space being linked to an existing JIRA project on creation.
If the name of the space in the "new space" dialog contains the word "documentation", Confluence creates a set of software project pages for the space in Italian, e.g. "Requisiti Prodotto" (product requirements).
A workaround is to not include the word "documentation" when creating the space, but to edit the name to include this once the space has been set up (after the project pages have been created in English).
Is this a bug? Is Confluence guessing the language for the newly-created space from the text of the specified space name? Have I missed a configuration setting somewhere in Confluence to always have space page titles be in English?