The mentioned select should be built based on the chosen language in the same form.

Steps to reproduce:

1. Create ger-DE language.
2. Add the following site accesses configuration:

           list: [site, test_en, test_ger]
           groups:
               test_group:
                   - test_en
                   - test_ger
               site_group: [site]
           default_siteaccess: site
   

           test_en:
               languages: [eng-GB]
           test_ger:
               languages: [ger-DE]
           test_group:
               content:
                   tree_root:
                       location_id: 42 #ID of Ibexa Platform folder
   

3. Create folder in eng-GB under Ibexa Platform folder.
4. Translate the mentioned folder to ger-DE language.
5. Try to add custom alias, choose ger-DE as language and choose test_ger Site Access below.

Result:
You are only allowed to choose site or test_en Site Access.

Expected result:
Based on the language chosen you should be able to select different Site Accesses. Select options loaded via ajax may be needed here.