How to keep plugin translates after updates?

Question

http://wordpress.org/extend/plugins/bbpress/ bbpress' language folder ( wp-content/plugins/bbpress/bbp-languages ) have that warning:

/**
 * Do not put custom translations here. They will be deleted on bbPress updates.
 *
 * Keep custom bbPress translations in /wp-content/languages/
 */

Actually this is not a new problem and yes they are right. This is big problem if you are using wordpress plugins with a non-english language.

Basically i translated bbpress and created .po and .mo files. Files are working if i put them in their normal wp-content/plugins/bbpress/bbp-languages folder. But as above warning says that will delete when update. But problem is, translating is not working if i put them in wp-content/languages/ folder as suggested in bbpress.

I think there must be a hook or something i can activate it but what is best solution for it? Simply i want to keep plugin language files in wp-content/languages/