Languages which use either of the above mentioned scripts, or scripts derived from them, are Arabic, Gilaki, Hebrew, Ladino, Pashtu, Persian, Sindhi, Syriac, Urdu, Yiddish and several others. They are used mostly in the Middle East and central Asia.
A related term is "Bidi text" meaning bi-directional text -- how RTL and LTR text displays together on the same page. The short solution to problems related to this is simply keep RTL and LTR separated by line breaks. Some strange and seemingly impossible things happen with a mixture of the two -- the oil/water analogy is a good one.
The problem really centers around the necessary HTML and Wiki markup such as adding [[links]](LTR characters) around RTL text. These problems are especially pronounced in a web editor field, and though they can be mitigated somewhat by making edits in a separate editor, (like VIM) this does nothing to deal with the fact that Wikis are 99% edited in the field provided on the webpage. And this field is for the most part within the dominion of the browser itself-- Mozilla, though pretty well developed has a couple of annoying bugs in its field that makes for a difficult time editing.
On English wikis, foreign encoded text is encoded in Unicode decimal -- so it is impossible to view the native RTL text, but:
This is sort of what it looks like -- text starts on the right, مرحباً|
making this a link (in the [[normal way]]) often at first looks like this:
]] Adding links like this[[ will usually result in a proper form: مرحباً|
So to get this ]] on an RTL text you type [[. Simple.
But often special characters like harakat and the final character may not respond well to being boxed in -- they will attach to the Wikibracket and break from the Arabic word, often causing a change in the way preceeding characters are displayed. (uses different characters for the final of a word)
Try it out at the http://ar.wikipedia.org/wiki/Sandbox --
Wikipedias using RTL
The he wiki seems to have been going along fine for a while--but some insights from our He.wikipedia Wikipedians will be useful.