[BUGFIX] Fallback to default language within internal linking suggestions if All Languages is selected, changed language error message

[RinyVT]

Summary

This PR can be summarized in the following changelog entry:

• In [BUGFIX] Show warning on linking suggestions when content element is set to 'All Languages' instead of fatal exception #544 an error message was introduced when a content element has "All Language" as the language setting. This PR changes the behavior to use the default language instead, and only show the error when getting the language from the site configuration fails.

Relevant technical choices:

• Use getDefaultLanguage of Site when sys_language_uid is -1.

Test instructions

This PR can be tested by following these steps:

• Pull branch
• Set a content element to "All languages"
• Check if the internal linking suggestions are now based on the default language

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #554