Update notification not shown for non-core locales with the same language as core locales (e.g. en-GB & en-US)

Environment

• OS version: Windows 10
• Browser version: Chrome 80, (did not check other browsers since I don't think it's browser specific)
• Extension version: UI release 2019-3
• Last working version: n/a, updates notification was implemented in UI-release-2019-3

Steps to reproduce

1. set the Chrome locale to en-GB (on chrome on windows: settings -> advanced -> language -> add English (United Kingtom) -> hamburger menu next to language -> display google in this language -> restart browser)
2. trigger the update notification (check #747 (closed) for ways of doing that)

Observed behavior

Update notification is not triggered

Expected behavior

Update notification should be triggered:

• ABP icon in the toolbar should animate
• update notification should be shown in the popup

Further information

• updates page was implemented in #562 (closed)
• core locales defined here: https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/-/merge_requests/463/diffs#2de6998fe8499a33b505d255d4b69095957b4b60_0_2

FWIW: I think the issue might be here: https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/-/merge_requests/463/diffs#1538038c067041e30e7013d531ddeb2815b21fd5_88_101

In the case of en-GB, the language is set to en, which is then compared to the coreLocales. There is no en in coreLocales, only en-US, so the notification is not triggered. (Changing the en-US to en in coreLocales (https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/-/merge_requests/463/diffs#2de6998fe8499a33b505d255d4b69095957b4b60_0_2) caused the notification to finally get triggered, but I obviously don't know if that's the right way to go about it.)