Email locale should honour UI language for new registrations

[claudiushenrichs]

Current Behavior

Registration emails are always sent in the forum's default language even if the visitor picked a different UI language before

Steps to Reproduce

1. Have a forum set up with at least one non-English language pack
2. Have the default language set to English
3. Not being signed in (e.g. using incognito browsing) select a non-English language via the language picker
4. Click "Sign up" and follow the registration flow
5. Observe the language set email is sent in => Always English

Expected Behavior

The email language locale should reflect the user choice taken via the Ui language picker

Screenshots

No response

Environment

• Flarum version: 2.0.0-rc.8

Output of php flarum info

No response

Possible Solution

No response

Additional Context

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