Generation of documents requires a locale so that the projectdoc Toolbox knows the language to use to generate property names and section titles. The selection of the locale is defined by the site locale.
To change this there must be the notion of a locale per space or even page. A macro can at any time render text dependent on the locale of the current user. But on page creation the generated text need to follow the locale of the site. If the locale of the user would determine the language of the generated page, a space would have documents for different languages.
Currently it is not supported to have the locale be determined by a space or page configuration (see Controlling Locale/Language for Space and Page Blueprints in Confluence).
A possible workaround: The templates a blueprint is based on may be translated at space level. This way the template of a blueprint for the site locale "English" could be translated in a space to e.g. "German" (see Blueprints Cannot Be Configured to Use Non-English Languages).
Please refer to Localization for more information on this topic.