OS X supports a large number of world languages, but if you only understand one language, you’re hauling around many megabytes of files you don’t need. Many Mac users try to free up some disk space by using a utility that removes versions of files for languages they don’t want. This has given rise to a class of utilities that hunt down and delete all the files for specified languages. Some of these utilities are:
Delocalizer [Note: This software appears to be discontinued. (June 2011) ]
Macaroni
Monolingual
Youpi Optimizer  [Note: This link is now dead. (June 2011) ]

Nearly all of my applications run fine after delocalization, but there are a few that don’t like to have their localized resources deleted, and they refuse to run without them. These applications might not display a specific error message for this, so all you see is a general error message that leaves you guessing.

The applications that I know about that don’t like to be delocalized are:
Adobe Illustrator CS2 (the 12.0.1 updater fails to install the update)
Adobe Acrobat 7 (Macaroni users update to Macaroni 2.0.6 or later)
Adobe Acrobat 8 (application will not launch, says “A required component was not found.”)

(NOTE: Macaroni 2.0.8 is now available, and according to its release notes, it will no longer delocalize any Adobe applications. This should prevent future Macaroni delocalization problems with Adobe apps.)

If you run a delocalizer and those applications or their updaters stop working, you must reinstall the application. Then you have a choice: Either see if your delocalizer utility has a way to exclude specific applications (a whitelist), or don’t delocalize again. Read on for why that second option might be realistic.

Nobody likes to have something not work, especially if it’s just one pesky app that breaks after delocalization when all the other apps are fine. And delocalization can benefit users with small hard disks. But you may not need to delocalize if you have an up-to-date computer. Today’s hard drives, now typically 250GB and up even on notebooks, are so large that a hundred or so megabytes of localized files are unlikely to be the major cause of a full hard drive. If your Mac hard drive is filling up, you’ll probably get a lot further by moving unneeded movies, photos, and music to another drive. One hour of miniDV format video from is around 13GB, many times larger than all your localized files. Also, the digital audio and graphics samples that come with multimedia applications such as GarageBand, Apple Soundtrack, and Apple LiveType take up several gigabytes on their own, so you might want to clean house there unless you need those samples.

Why would an application fail after delocalization? In many cases an application will use a checksum to verify its integrity. It knows what its file size was when originally installed, and if it notices that there’s a discrepancy the next time it launches (because you removed stuff), it won’t run. It does this because a size discrepancy can indicate that the application code has become corrupted or that malware has infected and modified the application for its own nefarious purposes. Those are some of the reasons why an application may be built to be extremely suspicious when its size changes, to the point of shutting itself down. The developer wants to make sure that the application won’t do any damage to your files.