WordPress.org

Which storage is better / faster?

The exact performance will depend on your configuration but in my testing APC is a tiny bit faster than shared memory which is a bit faster than plain file storage. Check out the plugin homepage for some graphs.

What is APC (Alternative PHP Cache)?

"The Alternative PHP Cache (APC) is a free and open opcode cache for PHP. Its goal is to provide a free, open, and robust framework for caching and optimizing PHP intermediate code."

It is not usually installed by default.

What is SHMOP (Shared Memory)?

SHMOP is a PHP interface to handle Unix shared memory segments. It should be available by default on Linux/Unix servers. On Windows it may be possible to enable in PHP.ini.

On some older systems (and Windows) shared memory may have problems when multiple languages are cached. (If Open slots in the debug storage information keeps decreasing you should choose a different storage type.)

How do I make files writable?

http://codex.wordpress.org/Changing_File_Permissions

File storage write files to wp-config/cache/l10n-cache/. These directories will be created automatically if wp-config is writable at first use. Once these sub-directories are created wp-config should be kept write protected.

I want to change where the files are stored

Currently you'll have to do it by hand. Find it at the top of wp-content/plugin/cache-translation-object/cache-translation-object-file.php