As a follow-up T204797: [Regression 1.32.0-wmf.22] ParserOutput::getLanguageLinks returns invalid values (Undefined index from ApiParse and LinksUpdate) to, the sidebar-fallback option is still "broken" and will cause warnings and errors.

As this was an experimental pet project, I don't have time to conduct extensive refactoring.

Some possibilities how to solve this:

1. Drop the sidebar-fallback option. I feel this mode is useful for getting the feature deployed, so probably not a good solution.
2. Try to add more bandage to it to avoid the warnings and errors.
3. Do a refactor of the interlanguage links code to make it easier to use and more flexible.

Third option is probably the best, and a refactoring is long overdue here. In my opinion it should have already happened when compact language links was worked on.