Add English (United Kingdom) locale

[MarvinZeising]

Description

With the rollout of the plugin_studio_migration feature flag, the form edit view is now not embedded within the admin frontend anymore, but the experience studio instead.

As it seems, the language detection coming from this SDK was overwritten by the available languages in the frontend. Since the new Studio doesn't have that, it now actually uses this SDK's list of languages and can't resolve en_GB.

Related Issue

https://staffbasehq.slack.com/archives/C093GKLJ7B5/p1765449075066749

How Has This Been Tested?

• dev it
• create a form with enGB locale
• activate the plugin_studio_migration feature flag
• open that form
• should display correctly
• try to remove enGB and add it again
• should be available and work as intended

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING part in the readme.
• I have added tests to cover my changes.
• All new and existing tests passed.

Test plan:

[github-actions]

size-limit report 📦

Path	Size
src/main.js	6.62 KB (+0.39% 🔺)

[Magnet-js]

Im not sure but I can still reproduce the issue on dev.

[staffbase-actions]

🎉 This PR is included in version 3.0.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀