Isn’t it dangerous to modify the binary data of applications?
Removing code that will never be run on your machine is safe. However, a very small number of applications may not function properly after they have been slimmed down, because they may check themselves to enforce anti-piracy measures. Xslimmer takes a number of precautions to avoid putting you in a position where an application cannot be used after being slimmed down. It provides a backup mechanism that can be enabled from Xslimmer’s Preferences, automatically storing a copy of the applications before they are slimmed down. It also provides the means to “blacklist” applications that you don’t want ever to slim down. The blacklist is automatically configured to ignore some popular applications known to perform integrity checks on themselves.

We recommend that you enable the backup option if you intend to reduce the size of an application that you will not be able to reinstall.

Just be careful with these apps if you are running even one thing through Rosetta: they can hose your Rosetta setup to the point where you need to reinstall OSX. The problem is that Rosetta is so transparent that you might not even realise that you are using it (Hint: if you use MSOffice 2004 for Mac, or anything from Adobe older than the end of May 2007, you are a Rosetta user). From what I’ve seen on the website, xslimmer seems a little smarter – it gives per-app control and you can avoid “slimming” any system files. Monolingual just starts processing your disk while you cross your fingers.

But if you are still on a PPC machine, dumping the Intel code should do no harm.

Removing unneeded languages, which both these apps do, is fairly safe, just remember to keep both the base “English” as well as “US English” or “UK English”.

And of course, you do need to remember to “slim” your apps after every upgrade.

So its true that you can reclaim disk space, but there is a reason to avoid it. Some applications, World of Warcraft is an example, used PowerPC code even though the main program was a UB. Some users removed PowerPC code and WoW stopped working. I would not remove PowerPC code on Intel for this reason alone, if your apps break, you have to reinstall, not so much fun. Removing languages you don’t need is a much safer bet, and in my experience you get GBs of space back.

As for RAM usage, that’s just wishful thinking. The program loader only loads the binary stream for your architecture, or if Intel isn’t available, a PowerPC binary stream. There is no RAM savings because the bits are never loaded. Even though OS X Applications look opaque to the user, they are in fact a collection of files. Right-click on any app and select Show Package Contents and see how far the rabbit hole goes.