English is his native language. He may have learned Vai, but only as a secondary language.
Some Vai, many of whom are Muslims, are literate in three different languages and scripts: Vai, Arabic, and English. The script has been kept alive largely as a matter of pride. It is not taught routinely to Vai children but is transmitted by knowledgeable elders to youngsters who express a desire to learn it.
It seems to me that this is akin to the idea that a Catholic on trial would require a Latin translator.
Just an incredibly stupid performance by the judge and court.