So, where exactly do all those .localized files live?

The language files are components of application packages. A typical application or system component is made up of many files and folders that are packaged together and appear as a single standalone program. In reality the application may contain thousands of separate files and folders. The packaging process makes it all appear as though it were a single file. This also makes fixing problems much easier because of the modular nature of application packages.

It's relatively easy to see this by selecting any application and then CTRL or RIGHT clicking to open the contextual menu. Select "Show Package Contents" from the contextual menu. A window will appear with a single folder named "Contents." You can then navigate through the package in the Finder. Eventually you will find a folder named "Resources" in which a bunch of folders will be found with the extension ".lproj", e.g., English.lproj. These are the language localization folders.

Be aware that some applications don't like their language localization files removed. Adobe products in particular. Removing their localization files will cause Adobe updaters to treat the modified package as an illegitimate version and refuse to update the package.