i18n: use correct ISO 639-1 code for Korean with backward compat

i18n: use correct ISO 639-1 code for Korean with backward compat

recently the proxmox-i18n repo got a fix where we moved the files for
Korean to the correct language code, i.e., from previously wrong used
kr (Kanuri) to the correct ko (Korean).

This loads the correct ExtJS locale and is less confusing for our
Korean speakers, but we still want a clean transition for those that
have still the 'kr' value set in their language cookie.
Note that this transition only happens when the user opens the
language selector, as otherwise we do not have the product-specific
cookie name available, so a better transition would need to happen in
the per-product UIs.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>