fix: components not updating correctly when changing language /2

Fixes :
- lacking i18n.global.locale.value in datetime.ts
- remove a test in i18n.ts that was always false in past versions of the code because i18n.global.locale is an object and lang is a string
- correctly type createI18n with <false> for legacy to solve wrong TypeScript error (see https://github.com/intlify/vue-i18n/issues/785, https://github.com/intlify/vue-i18n/issues/889#issuecomment-1017250573)

Fixes: #1597, #1772, and possibly other related issues.

src/filters/datetime.ts

@@ -79,7 +79,7 @@ function formatDateTimeString(
   return format.format(parseDateTime(value));
 }
 
-const locale = () => i18n.global.locale.replace("_", "-");
+const locale = () => i18n.global.locale.value.replace("_", "-");
 
 export {
   formatDateISOStringWithoutTime,