Check if other locale exists
You can query for the same entry in another locale by explicitly setting the locale parameter.

{% set de = craft.entries.id(entry.id).locale('de').first() %}
{% set en = craft.entries.id(entry.id).locale('en').first() %}

German:  {{ de ? 'OK' : 'nope' }}
English: {{ en ? 'OK' : 'nope' }}