Unfortunately, there does not appear to be a way to change the default language away from EN-US, in the wordpress.org repository.
An email responce just recieved from plugins[at]wordpress.org has confirmed that the default of EN-US is an intentional policy process.
The submission is done in English US and then you will be able to translate it to any other language.
Which is a ‘Whats That For (WTF)’ eyebrow-raising assumption for a policy & process; because, it implies that all EN plugins are written in US style spelling and phrasing for the entire english language; nope, thats obviously not right.
However, the wordpress.org policy process for language translation does really appear to be setup as EN-US as the key. The translation glossaries for EN-GB Glossary & EN-AU Glossary have way to many ‘z’s and not enough ‘u’s in the ‘Item’ column. The ‘Item’ column being the EN-US term used as the official translation key.
Hence, it looks like the approach to anchor to just one flavour of english (like EN-US) might have been chosen to reduce the day-to-day activity noise and practical tooling implications of so many world-wide languages. By not anchoring to one flavour, the translation multiplication of “from x to” gets pretty messy.
Action Recommended: As the plugin author who is fluent in EN-GB, it looks like you might be able to become a ‘Translation Editor’ for your own plugin for the EN-GB locale. Have a read through ‘Plugin/Theme Author Translation Review/PTE Request’ for step-by-step instructions.
Outcome: This would at least open up a way to get rid of the yellow bar at the top of the English Chess Data (EN-GB) plugin page.