how safe is it to use the application monolingual

From what I have seen here in the forums, some users are able to use it safely, but others lack the skills to do so and wind up deleting things essential for their os to run properly, so they have to reinstall it.

Do you really have a space problem? If not I would recommend you avoid this and other apps that mess with your system in that way.

FWIW, one of the problems was and still may be, is certain software will not run or will crash if the standard set of languages aren't installed. There is little to gain and much to lose.

As mentioned, you need to be careful with it since certain apps will fail to launch if any language files are removed from them, even if it's not a language you're using.

I do use it myself since it can open up a lot of disk space, depending on the apps you have installed. For Office 2016 alone, it frees up close to 1.5 GB of space.

To prevent Monolingual from processing apps that can't be touched, you add them to its preferences, and then turn off the check boxes next to them. Here's my current list.

The top four items are there by default. The rest I added. The Adobe Suite (especially CS6 and earlier) don't like having language files removed. Data Rescue and ColorBurst won't launch if processed. Quark will launch, but acts goofy.

Basically, if you're going to use it, make a full, restorable backup FIRST. Then run the app and test to see if any apps don't work afterwards. The typical problem being that they simply won't launch.

This was more useful a while back with smaller drives where any space you could save was a plus. With 2 and 3 TB drives, it's rather pointless since it's darn tough to run out of room on such a drive unless you're the type of person who collects thousands of hours of music files, or full length HD movies by the dozens.

So consider before anything else - do you really need 2 or so GB of space back?