English language selection generates HTTP500

[vRShop]

Hi, I have a problem with my shop 1.7.7 since start this week. When I try to change the shop language at the front end with the language selector I get an HTTP500 error.

The log shows:

PHP Fatal error: Uncaught PrestaShop\\PrestaShop\\Core\\Localization\\Exception\\LocalizationException: Unknown locale code: en-GB

There were made no changes by me to the shop system, only some days before I made an update to the shop system by 1-click-updater from 1.7.6 to 1.7.7 and I am unsure if this could be the problem.

I didn´t made a backup with the 1-click-updater but there are updates made from the whole shopsystem available.

Any help? Thank you!