**Problem:**
With T329655 and T338330 we have already improved the placeholders in the termbox. For now, the functionality is limited as placeholders can only show the `mul` values and no other languages from the fallback chain. This gives users a limited and somewhat distorted understanding of our language fallback chain.
**Solution:**
Use the language fallback chain to create the placeholders for missing values in the desktop termbox
* We allow language individual language fallback for Label and Alias placeholders. This allows placeholders to better represent the actual fallback chain so that users can more intuitively understand how (not) to edit the termbox.
* We can apply our normal language fallback per target language for this, but there is one exception: We stop the fallback chain with a hardcoded `mul` and not with the usual final fallback to `en` (as the `en` fallback is not an intended fallback, but more of temporary a last resort).
**Notes:**
* We still only allow placeholders with fallbacks for Labels and Aliases and not for descriptions (T313027).
* There is no need for live updates.
**Mockups:**
**Acceptance criteria:**
[] Every placeholder in the desktop termbox tries to fall back to the nearest available language.
[] viewing mode
[] editing mode
[] We stop the fallback chain at the hardcoded `mul` (so we do not apply the usual final fallback to `en`).
**Original:**
{T307274} by @Bugreporter