List of steps to reproduce (step by step, including full links if applicable):

• Start adding a statement on a Commons file
• Select a monolingual text property, e.g. Sandbox-Monolingual text
• Click "Select language" and enter the code sjt (some others with the same problem: pyu, sjk, gml, gmy)

What happens?:

The name of the language is shown in the input field, while the search results say "No results found."

Pressing enter will select the language, allowing the statement to be added, but there is no way to select the language with the mouse.

What should have happened instead?:

The language should be shown in the search results.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc.:

As far as I can tell, this affects languages which have been enabled for monolingual text in WikibaseContentLanguages.php but have not been added to langdb, since codes such as cnr, nr and tzl which are in both files work fine.