TYPO3 Core

Use case :

Our servers don't have access to TER or any other sites so we manually copy languages files in /typo3conf/l10n/fr/

In 7.6, using the BE module Languages throw an error when clicking :
"Oops, an error occurred!
Could not access remote resource https://repositories.typo3.org/mirrors.xml.gz."

The first time we need to add French as a language we're forced to edit LocalConfiguration.php by hand and change

'availableLanguages' => [],

To :

'availableLanguages' => [
'fr',
],

Showing the error "Could not access remote resource https://repositories.typo3.org/mirrors.xml.gz." is ok but the module should still be usable since manually added language is possible and in our case were present physically in /l10n/.