They use the Wikipedia featured articles as source for summary texts, since they are (i) well-written, (ii) comprehensive, (iii) well-researched, (iv) neutral, and (v) stable according to the Wikipedia featured article criteria 2 .
we were not able to retrieve results for a significant number of sentences with the search engine in particular for German sentences (English: 10,102 (14.20%), German: 9,325 (41.81%) sentences with an empty search results).