fix(vue-demo-store): extract lang code from accept-language header

[mkucmus]

Description

Browser can have a setting with multilanguages allowed as an accept-language header "de,en-US;q=0.7,en;q=0.3";
the order sticks to the priority.
thanks to this we can find the first matching language code from the string, and then, the language code can be used by

new Intl.NumberFormat('de', {
      style: "currency",
      currency: 'EUR',
    }).format(123)
'123,00 €'

Type of change

• 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 not work as expected)

Screenshots (if applicable)

Additional context

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
frontends-demo	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 12, 2023 7:52am
shopware-frontends-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 12, 2023 7:52am

[BrocksiNet]

Tested 👍

At the moment also when you link from Twitter to the demo page the page is broken. With that fix, it was working as expected. We should merge this fast.