Internationalize sponsors

@k1r1ll or @benni you have to add the translations to the pelicanconf.py of each project. (schul-frei.org, teckids.org/teckids.edugit.io) Maybe below SPONSOR_LOGOS and in this format:

SPONSORSTEXTS = { "de": "Danke...", "en": "Thanks...", "fr": "Merci..." }

templates/article.html

@@ -86,6 +86,11 @@
 </section>
 
 {% endblock content %}
+
+{% block sponsorstext %}
+{{ SPONSORSTEXTS[article.lang] }}
+{% endblock %}
+
 {% block languagefooter %}
 {% import 'translations.html' as translations with context %}
 {{ translations.translations_for(article) }}