The people who localise for their language at Betawiki, all have an ability to understand English. In many ways, the first person to start the process of localisation is a pioneer. The objective is often simply to meet the requirement for a new project. For new languages, when 50% of the "most used messages" are done, the user interface becomes available in the Incubator and all other WMF projects.
For some languages the localisation has become a continuous process; the usability for these languages becomes as good as it gets. When more and more work is done, a need arises for other activities. Proof reading, the use of consistent terminology require other skills, different skills. English is for these people not as much a requirement as it is for the people who do the primary localisation work and Betawiki needs to provide great support for the other languages.
A good example of a well supported language is Persian. The language has had consistent support for quite some time. It is of a quality where MediaWiki is usable for use in education or where native hosting in Persian can be offered. Betawiki allows its users to change the interface to right to left support, a gadget still missing for Commons.
To support the people who are not comfortable with English, the Betawiki user interface itself has to be localised. When the most basic localisation is completed, proof reading and improved terminology are activities that become increasingly important. The Persian effort demonstrates how good it can get..