Respecting users language when formatting dates

When formatting dates the users locale setting is used but not the language.

Proposed change

Keep using users locale to determine date formats.
But use the users language for translating the words (month and weekday names) in the formatted date.

Example

• Given
  • User language "German"
  • User locale "English"
  • Date '2026-03-02'
• When
• Formatting date equivalent to Intl.DateTimeFormat(.., { dateStyle: 'full'})
• Then (currently)
  • Resulted is Monday, March 2, 2026
• Then (with proposed change)
  • Resulted is Montag, März 2, 2026

Note the result is not 'Montag, 2. März 2026' which would be the German format for { dateStyle: 'full'}.
Montag, März 2, 2026 is still the English format but with German words.

Affect components

• NcDateTimePicker
  • uses date-fns for formatting
• NcDateTime
  • use useFormatTime
    • uses Intl.DateTime

Background

Review comment https://github.com/nextcloud-libraries/nextcloud-vue/pull/8644#issuecomment-5103248212

Start with the NcDateTimePicker component, which uses date-fns, and NcDateTime through useFormatTime, which uses Intl.DateTime. Trace how each receives the user's locale and language, then verify both preserve locale-based date formatting while translating month and weekday names with the language. Use the German-language, English-locale example as the completion check.