Problem:
While investigating T170424 I deep dove (description at T170424#5802420) and found that most of our UI calls to the API are probably using the backward compatibility layer.
In order to solve T170424 and get more relevant and translated errors to our users we should specify an errorformat so that the newer code for error output in the MediaWiki API is used.
Example:
Look at the first error to appear in the JSON output
Acceptance criteria:
- All Wikidata / WMDE owned UI code calling APIs should specify an errorformat and a uselang parameter