I have put the source code of language plugins, and the essential header file(filter.h) for writing AviUtl's plugin (with comments translated to English) on Google Code (using Git):

https://code.google.com/p/aviutl-base-i18n/

Note that these files can only translate the BASE part of AviUtl UI, not for Advanced Editing NLE (which requires reverse engineering)

For anyone interested in translating the UI, just grab one of the .rc file, and translate all the double quoted text.

If you don not know how to use Git or just do not want to bother other technical stuff, just post your translated file here.

If your are interested in writing Plugins or using it with OpenCV, I have some codes for full plugins here for your reference:

Motion Tracking with OpenCV: http://mavericktse.mooo.com/wordpress/archives/1534

Subtitle removal with OpenCV inpainting and canny edge: http://mavericktse.mooo.com/wordpress/archives/1708

PM me if you want to be a member of this project.