@f4dhli or ztutorial:
I want to take part in translation as a validator. I want to be a translator at a local support forum moderators for Indonesian.
We already have validators for WP.com. There's no limit as to how many validators a language could have but 2 or 3 people are more than enough since to many validators will usually lead to inconsistent & ununiform translation. See this thread for more information.
I see that you are already a moderator in Indonesian support forum and there's no need for local support forum to be translated.
I’ve tried changing the language in http://translate.wordpress.com/projects/wpcom/id/default but it does not work.
If what you are asking is why your translation does not immediately applied to wp.com, it is because your translation have to be approved by a validator before it is used in wp.com. It may takes some time before your translation (if approved) is used in wp.com. By keep contributing to the translation you already helping us.
The Indonesian wp.org team is looking for vounteer to translate WordPress for Windows Phone & WordPress for iOS. You could contact them here if you're interested.