GJ has spent alot of time in the Pacific islands apparently and knows the local languages very well. especially Tuvaluan. He and his wife were critical in the translation of literature into this language and they have even written a books on the subject.
This experience is his qualification....
http://www.jw.org/en/publications/magazines/w20150815/let-many-islands-rejoice
Here's a good quote...
In 1980 the branch office assigned us to work as translators—a work for which we felt totally unqualified.