Add support for language exclusion in sitemap.xml

This PR adds the ability to exclude specific languages from sitemap.xml generation.

### What’s new:
- New property `DisallowLanguages` in `SitemapXmlSettings`
- `SitemapModelFactory` and alternate URL logic now respects this setting
- Default behavior remains unchanged (no exclusions)

### Why:
In some scenarios, site owners may want to:
- Prevent certain language versions from being listed in sitemaps (e.g. test/staging content)
- Avoid conflicting SEO signals when the same languages are excluded in `robots.txt`

This setting provides fine-grained control and separates concerns between `robots.txt` and sitemap configuration.

Closes #7777

Author: Jan Nielsen

src/Libraries/Nop.Core/Domain/Common/SitemapXmlSettings.cs

@@ -71,4 +71,9 @@ public SitemapXmlSettings()
     /// Gets or sets the wait time (in seconds) before the operation can be started again
     /// </summary>
     public int SitemapBuildOperationDelay { get; set; }
+
+    /// <summary>
+    /// Disallow languages
+    /// </summary>
+    public List<int> DisallowLanguages { get; set; } = new();
 }

src/Presentation/Nop.Web/Factories/SitemapModelFactory.cs

@@ -887,7 +887,8 @@ var name when name.Equals(nameof(ProductTag), StringComparison.InvariantCultureI
 
         var updatedOn = dateTimeUpdatedOn ?? DateTime.UtcNow;
         var languages = _localizationSettings.SeoFriendlyUrlsForLanguagesEnabled
-            ? await _languageService.GetAllLanguagesAsync(storeId: store.Id)
+            ? (await _languageService.GetAllLanguagesAsync(storeId: store.Id))
+            .Where(lang => !_sitemapXmlSettings.DisallowLanguages.Contains(lang.Id)).ToList()
             : null;
 
         if (languages == null || languages.Count == 1)