**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 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 translatewiki language fallback per target language for this, plus hardcoded `mul`, but we do not apply the usual final hardcoded `en` and we also do not consider user-specific fallbacks like Babels.
**Scope of this task:**
We want to create a low-effort version of this functionality so that the Wikidata Community can test it on Test Wikidata. Only if the test goes well, will we invest in an optimized implementation to be released on Wikidata Proper (e.g. possibly initially as a beta feature).
This **low-effort version** comes with the following limitations:
* PHP-only implementation: This means there will be no JS-based live updates after editing. Editors need to reload the page to see their changes reflected in the placeholders.
* Ignore ULS-added languages: Wikidata adds languages from ULS (for non-logged-in users, for logged-in users with fewer than 4 UI/Babel-specified languages), it is okay to only apply MUL placeholders for these (where available).
* No optimizations needed: This will for now only run on Test Wikidata. So the implementation can still be optimized for Wikidata Proper at a later time.
Ideas for full implementation (not in the scope of this task):
* To get an efficient JS implementation (including the ULS-added languages), we could include a cacheable lookup table for the fallback chain among the initial page assets.
* The difficult parts of the full implementation are independent of the UI stack (so the work will in any case be helpful for the new editing UI).
**Notes:**
* We still only allow placeholders with fallbacks for Labels and Aliases and not for descriptions (T313027).
**Mockups:**
**Acceptance criteria:**
This task is the last piece for v0.4 which will allow the community to test our UX changes before we work on it more for a full release. This will be released only on Test Wikidata and the functionality in this task is purposefully limited to a PHP-only version for testing, see "low-effort version" above. The goal was to reduce the initial complexity, so in case something turns out harder than we initially thought, please check in with Manuel.
[] Every placeholder in the desktop termbox tries to fall back to the nearest available language (we only consider fallbacks from translatewiki plus the hardcoded `mul`, not the hardcoded `en`, and no user-specific fallbacks from Babels).
[] viewing mode (only Labels)
[] editing mode (Labels and Aliases)
[] We stop the fallback chain at the hardcoded `mul` (so we do not apply the usual final fallback to `en`).
**Original:**
{T307274} by @Bugreporter