Fixed customers receive English emails from Stripe if site uses formal German locale.

[sun]

Fixes #2114

Changes proposed in this Pull Request:

If a site uses the formal German locale (de_DE_formal), then the locale is not sent to Stripe. This causes all emails from Stripe to be in English instead of German.

Testing instructions

1. Configure the site locale de_DE_formal in the WordPress language settings.
2. Place an order.
3. The emails sent from Stripe should be in German.

Notes

• Stripe does not send emails when testing/sandbox mode is enabled.

• Testing an array key lookup does not make sense. Therefore no automated tests are added.

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)

[a-danae]

Hey @sun 👋 !

Thank you very much for this PR. It looks good to me. It'd just need the changelog entries updated to be merged. I opened another PR - #2675 - with these changes in case you're not available.

Apologies for the delayed response and thanks again!