The diversity of national and official languages is a key element impacting on the ability of ASEAN member states to harmonise their laws so there is a common approach to prosecution of international criminal activity. Such an approach is critical as the ASEAN Economic Community moves to greater integration. The paper briefly describes the importance of language in understanding legal concepts and then describes the variety of legal systems in place across ASEAN which, other than the case of Thailand, are vestiges of their colonial past. The paper discusses three possible models for harmonisation/cooperation, namely: a set of model laws; accession to an international treaty; or an agreement to cooperate. In the case of Brunei Darussalam, Malaysia, the Philippines and Singapore where the legal systems use the English language, all three models could be used. For the other six countries, because of their diversity of language it is argued that the set of model laws is not appropriate. The preferred option is a treaty or conventional which sets out the scope and minimum requirements to be included in the local law and the obligations to cooperate with each other. The Convention on Cybercrime (Budapest Convention) is suggested as a possible model.