It was to decide whether or not the evidence provided by Collins was legal because Armstrong was not a parent or lawyer. The final decision by Muirhead J and Bowen J states that evidence was legal because it was up to the judge to determine if evidence is admissible and in this case the initial trial judge found Armstrong to be a good interpreter and had acted in good faith, thus admissible.
What does this case establish?
Well from the ruling, it established that it is ultimately up to the judge to decide whether certain evidence is admissible. However as a side note (in a dissenting statement by Brennan J) he states that it was difficult to believe the confession of a child was given voluntarily and further when a child is from a different cultural background to the interviewer and has a social and educational disadvantage, a confession as such is worth even less.
Relation to Crime?
It shows that minority groups in society may be disadvantaged by the legal system (criminal justice system) due to educational, cultural and social barriers. This can be used in an effectiveness question.