User:Base/Scripts/ChangeContentLanguageOfPageAndItsSubpages.js

Note: After publishing, you may have to bypass your browser's cache to see the changes.

Firefox / Safari: Hold Shift while clicking Reload, or press either Ctrl-F5 or Ctrl-R (⌘-R on a Mac)
Google Chrome: Press Ctrl-Shift-R (⌘-Shift-R on a Mac)
Internet Explorer / Edge: Hold Ctrl while clicking Refresh, or press Ctrl-F5
Opera: Press Ctrl-F5.

var api = new mw.Api();

var token = mw.user.tokens.values.csrfToken;
api.get({
    action: 'query',
    list: 'allpages',
    aplimit: 'max',
    apprefix: mw.config.get('wgTitle'),
    apnamespace: mw.config.get('wgNamespaceNumber'),
    formatversion: 2
}).done(function(data) {
    $.each(data.query.allpages, function() {
        var page = this;
        api.post({
            action: 'setpagelanguage',
            title: page.title,
            lang: 'es',
            reason: "Page is in Spanish",
            token: token,
            formatversion: 2
        }).done(function(data){
            console.log(page.title + ' must have had content language changed');
        })
    })
});