Language and translations of menus and stories have to be carefully examined.
If you previously had decided to display only one language in your Portal but had started adding translations to your stories, these will no longer be displayed until you set the Portal to several languages (Portal Manager > Tools & Administration > Portal Settings / General Settings > Languages).
In order to start converting your Portal from one language to multilingual without displaying the ongoing changes to your visitors, you can proceed like this:
1. Activate the language
2. Create menu in your "main language" and choose "only one language" or deactivate it completely for the time being (Portal Manager > Website & CMS > Menus // edit or create // General Settings > Restrictions - Language)
3. Translate menu
4. Assign stories to your menu in your "main lanuage"
5. Start translating the stories
6. Remove menu assignment "only one language" or activate menu
You can still assign restrictions to user groups, for example you can display menus only for admins or employees and then choose to display it to everyone when you have completed the translation process.
From SF version 6.4 language assignments have become more sensitive - if a menu is enabled only for one language ("as the only language"), then submenus and stories in this menu will not be displayed in a language no longer available (= 404 error returns).