The reason for this behavior can be seen here: https://www.wikidata.org/w/api.php?action=wbsgetsuggestions&language=en-US. There is no language "en-US" in MediaWiki. Most code falls back to the default "en". But the API module that is responsible for suggesting properties does not.

I can see how confusing this is. Unfortunately I don't have an idea for an easy fix. The error message is neither in Wikibase nor in the PropertySuggester, but in the basic MediaWiki core API code.

• We could try to teach the GetSuggestions.php API module to accept unknown language codes, but that alone would not fix the issue. The API would not fail any more, but still return nothing. It would need to fall back to English, and I don't know if this is a good idea.
• We could try to validate the language code in the frontend JavaScript code before passing it to GetSuggestions.php. But since core seems to be fine with any string, I don't know how such a validation (against what?) would look like.