Language-Fallback on custom translations

Describe the Improvement

When a custom translation is missing for the current language the key is displayed. This could be optimized in two ways:

1. Fallback to same base-language (e.g if we're on en-US but only have a translation vor en-GB this would be better then nothing)
2. Fallback to projects default language
3. As a last alternative use the translation-key