The fact that you get the error is T154780 and a separate problem. But we've changed how various error messages are generated recently, this might be a regression.

Yes, I agree, the error is not the object of the bug, only the fact that the error message is bilingual.

The API is just passing through the error it gets from the underlying Upload code. The API has control of only the first bit, "Could not store upload in the stash (UploadStashFileException):" or "Nu pot stoca încărcare în spațiul temporar (UploadStashFileException):", which is the part that's being localized correctly.

The rest of the error is coming from the UploadStash class, specifically this code:

if ( !$storeStatus->isOK() ) {
    // It is a convention in MediaWiki to only return one error per API
    // exception, even if multiple errors are available. We use reset()
    // to pick the "first" thing that was wrong, preferring errors to
    // warnings. This is a bit lame, as we may have more info in the
    // $storeStatus and we're throwing it away, but to fix it means
    // redesigning API errors significantly.
    // $storeStatus->value just contains the virtual URL (if anything)
    // which is probably useless to the caller.
    $error = $storeStatus->getErrorsArray();
    $error = reset( $error );
    if ( !count( $error ) ) {
        $error = $storeStatus->getWarningsArray();
        $error = reset( $error );
        if ( !count( $error ) ) {
            $error = [ 'unknown', 'no error recorded' ];
        }
    }
    // At this point, $error should contain the single "most important"
    // error, plus any parameters.
    $errorMsg = array_shift( $error );
    throw new UploadStashFileException( "Error storing file in '$path': "
        . wfMessage( $errorMsg, $error )->text() );
}

You can see there at the end both the always-English "Error storing file in '$path':" bit and the wfMessage( ... )->text() bit that's going to be using $wgLang instead of the error language set in the API call.

The solution is to rework the various Upload*Exception classes to implement ILocalizedException (and do it correctly, pass the Message object as a parameter to the wrapper Message instead of converting it to text). ApiUpload is already set up to handle getting an ILocalizedException by outputting the exception's message directly without prefixing it with "Could not store upload in the stash (UploadStashFileException):".

Change 383324 had a related patch set uploaded (by Cparle; owner: Cparle):
[mediawiki/core@master] Rework Upload*Exception classes to implement ILocalizedException

https://gerrit.wikimedia.org/r/383324

Change 383324 merged by jenkins-bot:
[mediawiki/core@master] Rework Upload*Exception classes to implement ILocalizedException

https://gerrit.wikimedia.org/r/383324