All of the following information is available on the site run by Monolingual's developers at http://monolingual.sourceforge.net/. What I've attempted to do here is consolidate that information in a format that makes it (hopefully) easier to use and understand.
Before performing any of the following, make sure you have a complete backup of your system. Recovery is all important if something goes amiss.
If you've run Monolingual for the first time (or many times), then you've noticed that it leaves many languages unchecked that you would like removed from your system. You've also probably looked in Monolingual's preferences for a way to change which languages it removes and found nothing. That's because the languages Monolingual removes is based on the languages you have selected or deselected in OS X's System Preferences. So to start, open the System Preferences and click on the International icon (in Snow Leopard and later, the icon is labeled Language & Text). If necessary, click the Language tab. You will see something similar to this:
Click on the "Edit List..." button. In the resulting sheet that appears, uncheck all languages you don't need on your system. If English is your main language, it's extremely important that you DO NOT remove the subset language you need. If you have only English unchecked when you run Monolingual, all subsets of English will be removed from your system, REQUIRING a reinstall of OS X and pretty much all of your third party applications.
This is less likely to happen in later versions of OS X. In 10.3.x and earlier, most OS X components had U.S. English as the default language when you examine items with Get Info. OS X 10.4.x and 10.5.x are marked just English, or en. In Snow Leopard and later, access to removing, or otherwise marking languages via the Get Info dialogue box has been removed. But the basic premise is still to make sure you keep both English and your localized subset checked.
So if you are a U.S. English user, both "English" and "U.S. English" need to be checked. This also applies to Canadian, British, Australian or other form of English. Both the subset English language you used when installing OS X and the base "English" need to be checked. Since my usage is U.S. English, my final form looks like the following:
Click "OK". The Language settings will show the following. Take note of the orange triangle. To make sure the changes you've made are in effect, restart your Mac, or logout and log back in.
A very important part of the rearranging for English languages is to make sure the base English is ahead of any English subset. Small oddities can occur in OS X otherwise. Such as Font Book not showing glyphs for some fonts. If French is your preferred language, the order would be French, English, U.S. English. You rearrange the list by dragging and dropping the language names to the positions you prefer.
Now launch Monolingual. Since I've already set up my preferences for English and U.S. English in the System Preferences, Monolingual comes up with those two languages already unchecked. All others are checked for removal as seen below:
For non U.S. English users, these settings can become more difficult. Mostly because you can't be sure the third party application you installed has an English version for your local dialect, such as Canadian English. In which case, Monolingual may end up removing the only English resources in your application if it's U.S. English and it's checked for removal in Monolingual. In such cases, you would probably have to reinstall the application. To avoid that, go into the System Preferences and place a check on the boxes for all versions of English listed. There isn't a choice for New Zealand English in OS X, so Monolingual will list as below when launched.
You'll have to manually remove the check from the "English (New Zealand)" box each time you run Monolingual, but you can't get any safer than that for protecting the English resources. From experience, I can say this isn't necessary for U.S. English users. English and U.S. English should by the only choices you ever need unchecked. But do what you feel is best for you.
It gets yet more complicated for non English users. Especially for applications from smaller companies which may not have for example, French or Spanish language resources. If you only have French unchecked in Monolingual, you could find yourself with many non functional applications when the English resources are removed. For such users, it would best to leave both your first language and all English languages checked in the System Preferences. This assuming you can of course read English, but use it in an application only when you have to. That way, both your preferred language and all English languages will not be removed by Monolingual.
You can really just ignore the Input Menu tab. The amount of hard drive space you'd save by removing unwanted Keyboard Layouts and Input Methods is minuscule. It's simply not worth the time it takes to check the boxes. The Input Menu tab has been eliminated in later versions of Monolingual, as shown below.
Monolingual has had major changes made to it in the earlier releases to keep it from damaging Rosetta on Intel based Macs. It is also now smart enough not to remove architectures that your application would need to run. Here's three different views of that tab. The first is how it automatically appears on a dual 2 GHz PowerPC G5. The second on an Intel based iMac. The third in Mountain Lion. If you are running Snow Leopard or later, the option for Intel 64-bit is also automatically unchecked, as shown in the right hand image.
As of version 1.3.7, Monolingual will not remove any architectures from the framework files of the /System/Library/Frameworks/ or /System/Library/PrivateFrameworks/ folders. This prevents Monolingual from breaking Rosetta. In the case of Cocoa apps, Monolingual also knows not to remove PowerPC code from a non Universal OS X application. So even though all PowerPC choices were automatically checked in the Architectures tab on the Intel based iMac, it will not remove the PowerPC code for those apps that contain no Intel code. I've run Monolingual numerous times on an Intel based iMac and it has never damaged either Rosetta, Microsoft Office 2004, or any other application that is PowerPC native only.
This next part is probably the most confusing of Monolingual to use. And that's how to keep it from removing language files from certain folders. Programs I know to be rendered unusable when removing language resources are Adobe's Acrobat Professional and its associated Distiller. Neither of these programs will run if you remove any language files. With Adobe's CS line of applications, all will run fine (excluding Acrobat Pro and the Distiller) with the languages removed. But in the case of at least Illustrator CS3 (version 13), the program cannot be updated if any language files are removed. Even if you just remove the EULA files in the Legal folder within the Illustrator CS3 folder.
So how do you keep Monolingual from removing languages from these applications? Contrary to what you would think, by adding them to Monolingual's Preferences. Here's how. Open Monolingual's Preferences. Click the Add button. Navigate to the folder you want to exclude and click Open. The path is added to the list of Preferences. Now, turn the check boxes next to your new entry OFF. If you're running Adobe Acrobat Professional, you will also want to add the PPD folder for your printers, as Acrobat won't run even if just the foreign language printer PPDs are removed. Your preferences will look something like the following, depending on what folder(s) you added. The first four are there by default. Later versions of the Acrobat Pro and the Distiller have stopped putting drivers into the PPD folder shown in the image below. You no longer need to include that folder in Monolingual's preferences, depending on the version of Acrobat you are using.
Since the folders I've added are not checked, Monolingual will not remove any language files from the contents of these folders. The Adobe apps are particularly picky about having anything removed. So for any of them, CS through CC, no matter which application, do not let Monolingual process them. Even if you do, it actually removes very little from the hard drive, but causes updating issues. So leave all Adobe applications alone.
I don't know if it's a bug, or an intentional part of Monolingual, but any items on the Desktop will not have any foreign language files removed. If you want them cleaned out, you'll have to put the items in the Applications folder, or other folder where Monolingual will act on them. This is a bit of an advantage with Quark 8. You can have the unused languages removed from every part of Quark 8 except the application file itself. Quark will not launch if you let Monolingual process that file. Before running Monolingual, move the Quark application file, named QuarkXPress, to the desktop. When Monolingual finishes, put it back. Then open Monolingual's preferences again and add the Quark XPress folder to exclude it. You will only need to process that folder again if you update Quark.
Another minor bug is in version 1.3.8. If you were using a previous version of Monolingual, you must toss the old preference file before running 1.3.8, or it won't function correctly. To do so, go to the Preferences folder in your user account and delete the file:
What happens otherwise is that you can launch the program and start the process to remove the unwanted language files, but it will return almost immediately and declare that no files were removed. If you had previously added any folders to protect in Monolingual's preferences, you will need to add them again after removing the old .plist file.
The Adobe CS4 applications have their own new annoyance. This is completely unrelated to Monolingual, but is good information to have. I have found that if you remove the profile "sRGB Profile.icc" from the /System/Library/ColorSync/Profiles/ folder, the File Info dialogue box in Illustrator CS4 will come up blank! So will the entire Adobe Extension Manager CS4 interface. What this one particular profile could possibly have to do with the display of a dialogue box, I don't know. You also can't move that profile to the /Library/ColorSync/Profiles/ folder, or the Profiles folder in your user account. While the profile is just as active in either location, the CS4 applications mentioned will blank out if that profile is not specifically in the System's Profiles folder.
Depending on what you've installed on your Mac, Monolingual can recover a lot of disk space. Between the OS, the Adobe Creative Suites, Quark, Final Cut Studio and many other applications, Monolingual removed 1.6 GB of unnecessary files from the hard drive. That's 1.6 GB of data I don't have to continually back up or wait for to restore. Over the course of a year, that's a significant savings of time.
Run Monolingual any time you've updated the system, an application or other components to keep your hard drive as trim as possible.