Topics and Answers don't have default language set

[lucaa]

To reproduce:

• set the language of the wiki to french (single language)
• create a new forum
• create a topic
• create an answer on this topic

Expected result: all the 3 pages created for these 3 entities have the default language set to french.

Actual result: the forum page is set to french, while the topic and answer pages have an empty default language.

Note that this is important because it impacts the indexing for solr search of the content in these 3 pages. In the situation described by this issue, the content is expected to be indexed as french language.