Almost every computer system sold in the last ten years supports Unicode, which is essentially just one single large character set that supports most characters used in all languages, and will in time, in theory, support every character used in any language.
See the offcial website of the Unicode Consortium at http://unicode.org/ for full information.
Non-Unicode programs are those that do not (yet) support Unicode. They instead use the smaller 8-bit character set which was the default character set in earlier versions of your operating system.
Changing to a different language within an application should normally only affect that single application. If you change to another language not supported by the current old-style character set, this may change your keyboard. Or if you are changing to a language that uses a completely different character set, that may change more.
Most operating systems, when you make such a change, provide a way of quickly changing back so that, for example, you can use Arabic and English on the same machine, switching between one and the other.
Without knowing exactly what operating system you are using and what version of the operating system, and what exactly you are trying to do, I can’t be more specific. But normally changing your system to handle another language for non-Unicode applications should not cause any problems, unless the application cannot handle that language. For example, not every older Word Processor can handle Arabic.
Normally, if your change does not prove satisfactory to you, you can always change back with no problems.
By the way, you can normally change your keyboard driver to use the keyboard for a different language. This will affect all programs, whether Unicode or non-Unicode. You can also change the default 8-bit character set. That will only affect non-Unicode programs. But it will affect all of them.