TYPO3 Core

our servers are behind a firewall. due to some changes we had the situation that our installations missed the language files although the loading seems to work: in the BE and with the typo3 console you saw a progress bar slowly filling and a success message, but in the end there was no language folder.

after some inspection I got thi sinformation:
while the files are requested from https://extensions.typo3.org/fileadmin/l10n/... there is a redirect to typo3.org which was blocked. and as a result came an information page from the firewall (which could not be processed as language file).

It was hard work to find the error as the loading of the language files gave no hint about any error.
Please provide some meaningful error messages if an error occured while loading or unpacking the language files