Luckily, this is an easy one and nothing you’ve done wrong per se. A quick analysis of the situation should explain…
The “latest” version of the Packagist Composer Repository is out-of-date (2017-09-28). Therefore, not compatible with Magento 2.2, 2.3. Hence the error message you’re seeing.
If you look at the
dev-master version of the Packagist Repository, you will see that this branch was updated very recently (2019-08-07). It’s safe to assume that this update addresses the issue.
So, instead of specifying the “latest” version, you should specify the “dev-master” version when loading composer:
composer require cubewebsites/magento2-language-en-gb:dev-master
If you checkout the GitHub Repository, the only difference between the 2 versions is a line in the
composer.json that tells composer its requirements (i.e. the compatibility with Magento 2.2, 2.3).