Fix language generation for Forums and (indirectly) Posts. Fixes #349

The old versions (including the legacy Hadoop Datagen) had a bug where
the language of a Forum was selected as a random number between 0 and
the number of languages a person spoke (0, 1, or 2).

The fix is trivial: this random number should be used to *select* a
language from the list of languages spoken by the person (instead of
simply using this number as the id of the language).

Author: szarnyasg

src/main/java/ldbc/snb/datagen/generator/generators/ForumGenerator.java

@@ -64,7 +64,7 @@ public class ForumGenerator {
      */
     Forum createWall(RandomGeneratorFarm randomFarm, long forumId, Person person, long blockId) {
 
-        int language = randomFarm.get(RandomGeneratorFarm.Aspect.LANGUAGE).nextInt(person.getLanguages().size());
+        int language = person.getLanguages().get(randomFarm.get(RandomGeneratorFarm.Aspect.LANGUAGE).nextInt(person.getLanguages().size()));
 
         // Check moderator can be added
         if (person.getDeletionDate() - person.getCreationDate() + DatagenParams.delta < 0){
@@ -134,7 +134,7 @@ Forum createGroup(RandomGeneratorFarm randomFarm, long forumId, Person moderator
         // the hasModerator edge is deleted if either the Forum (group) or the Person (moderator) is deleted
         long moderatorDeletionDate = Math.min(groupDeletionDate, moderator.getDeletionDate());
 
-        int language = randomFarm.get(RandomGeneratorFarm.Aspect.LANGUAGE).nextInt(moderator.getLanguages().size());
+        int language = moderator.getLanguages().get(randomFarm.get(RandomGeneratorFarm.Aspect.LANGUAGE).nextInt(moderator.getLanguages().size()));
 
         Iterator<Integer> iter = moderator.getInterests().iterator();
         int idx = randomFarm.get(RandomGeneratorFarm.Aspect.FORUM_INTEREST).nextInt(moderator.getInterests().size());
@@ -273,8 +273,7 @@ Forum createAlbum(RandomGeneratorFarm randomFarm, long forumId, Person person, i
             albumDeletionDate = person.getDeletionDate();
         }
 
-
-        int language = randomFarm.get(RandomGeneratorFarm.Aspect.LANGUAGE).nextInt(person.getLanguages().size());
+        int language = person.getLanguages().get(randomFarm.get(RandomGeneratorFarm.Aspect.LANGUAGE).nextInt(person.getLanguages().size()));
         Forum forum = new Forum(SN.formId(SN.composeId(forumId, albumCreationDate), blockId),
                 albumCreationDate,
                 albumDeletionDate,