After fixing this plugin for calibre 5, I realised it would be easy to extend it to do something I have needed to do. That is to change the currently enabled metadata source plugins. The main need I have for this is to change to a single metadata source when working on that plugin. And then being able to change back to my usual set of sources.
In the plugin configuration I have added a second list to add menu items. This is to configure the options for changing the metadata sources. When you add an entry (click the plus on the right side of the list), you can edit the title of the menu item and the list of metadata sources to enable when the menu item is selected. To change either of these, double-click the column or click in it and press the F2 key. For the metadata sources, a dialog will be displayed with the list of installed metadata sources. Select the sources you want and press the OK button. Then press the OK button on the configuration dialog to save the changes.
When the button for the plugin is pressed, the menu will be built including the new metadata source changing items. Clicking on one of these items will change the enabled metadata sources without any further prompting.
I don't know if any one else needs to use this, but, it will make my life easier
As well as this extra function, I have made the plugin translatable. The "default.po" is included plugin file for anyone who wants to do this.
If there are any problems with the new function, or the translations, please tell report them here.
Edit: Forgot to attach the plugin.