Document Encoding
The Kile editor allows you to read, convert and save text in the character encoding your document needs. With this it is possible, for example, to use accented characters, such as are commonly used in Italian or French, directly in LATEX documents. Selecting the encoding for your document can be done in two ways:
One way to set the document encoding is by using the submenu Settings Configure Kile... Editor, where you can set the default character encoding for all files.
Set the default character encoding
Set the default character encoding
A second way to set the encoding for a document is to choose the desired encoding within the wizard to create a new document.
LATEX itself only understands ASCII, which represents a very limited set of characters. Hence, it is not possible to use accented characters directly. To use them nevertheless, a special syntax was invented: such as for example \"e for ë. The inputenc package is available to help you with this. It is included in the preamble using \usepackage[latin1]{inputenc}, where the optional argument specifies the encoding you would like to use (nowadays in most cases utf8). This tells LATEX to translate all of the ë's you wrote to \"e's before compiling. Please refer to the inputenc documentation directly for more information. Last but not least: remember to make sure that your file is actually saved in the same encoding you specified for the inputenc package!
This multitude of different character coding tables has created numerous problems: for example, you cannot write a course of Turkish in French without losing one language's special characters. There is general agreement that, sooner or later, everybody will switch to Unicode. There are many implementations of Unicode, and UTF-8 is the most successful in ; ® relies instead on the more cumbersome and less flexible UCS-2. Most distributions have already begun setting their default encoding to UTF-8, and therefore you may be very interested in using the utf8 argument to the inputenc package.

The ucs Package
If you don't have the ucs package installed, you can proceed as follows:
Get the ucs package from CTAN.
To install it, unpack the downloaded file and place it in a folder listed in your $TEXINPUTS environment variable. This can also be set inside Kile.

\usepackage{ucs}
\usepackage[utf8]{inputenc}

XeLaTeX
If you are using XeLaTeX, you can simply load the xltxtra package. It will additionally load all the required packages.

\usepackage{xltxtra}

CJK Support
Adding support for ideographic languages is quite tricky. However, once you are done with it, it will work quite well. Other than installing packages, there is some extra configuration work to do.
Your distribution might already have a CJK (Chinese, Japanese, Korean) package ready for you, so you might be saved the hassle of manually installing everything. Do check before going forward!
There is the possibility of using the ucs package in order to write short snippets of CJK text, but that option is seriously limited as it does not handle, among other things, newlines. Instead, we will install the complete CJK-LATEX package and make it work for both LATEX and PDFLATEX. A lot of this material has been inspired by Pai H. Chou's page about how to setup PDFLATEX.
Download the CJK package. Copy its unpacked files to an appropriate subfolder of $TEXMF, just as you did with the ucs package before (see The ucs Package). The files will be unpacked in a CJK/X_Y.Z folder; it is not important that you take them out, though it will probably be tidier and easier for you to maintain.
Now you have to download a font that supports all the CJK characters you need. You can choose any *.ttf file that covers them, but in this walkthrough we will use Cyberbit. Unzip the file and rename Cyberbit.ttf to cyberbit.ttf, since uppercase might confuse your system.
Place cyberbit.ttf in a folder together with Unicode.sfd, and generate the *.tfm and *.enc files with the command $ ttf2tfm cyberbit.ttf -w cyberbit@Unicode@. For some reason, sometimes this does not produce the hundreds of files it should. Should that happen in your case, you can download both *.tfm and *.enc files.
Place the *.tfm files in an appropriate folder, say $TEXMF/fonts/tfm/bitstream/cyberbit/; the *.enc files may be installed in $TEXMF/pdftex/enc/cyberbit/.
Now we need a map file to connect the *.enc files to the font. Download cyberbit.map and install it in $TEXMF/pdftex/config/.
Download another file, c70cyberbit.fd, and place it in an appropriate folder. You may choose, for example, $TEXMF/tex/misc/.
The last file we have to generate is a Type 1 font, necessary to read files generated with LATEX. Run the command $ ttf2pfb cyberbit.ttf -o cyberbit.pfb, and copy the resulting cyberbit.pfb to a folder such as $TEXMF/fonts/type1/cyberbit/.
Let's now place cyberbit.ttf among the fonts where LATEX can find it. You could place it in a folder named $TEXMF/fonts/truetype/.
Check the configuration file you find at $TEXMF/web2c/texmf.cnf, and make sure that the line mentioning TTFONTS is uncommented and points to the folder where you saved cyberbit.ttf.
To make it possible for PDFLATEX to use your CJK fonts, it is necessary that you add a line in the configuration file $TEXMF/pdftex/config/pdftex.cfg. Add map +cyberbit.map in the file to complete the configuration for PDFLATEX.
To configure LATEX so that you can produce files with CJK characters, you have to add a line in file ttfonts.map. The file might be in a folder named $TEXMF/ttf2pk/, but you will probably have to look for it. Append the line cyberbit@Unicode@ cyberbit.ttf to it.
Now, you only have to run texhash and the system should be ready.
To test whether your configuration is correct, you can try to compile this test file.

CJK Troubleshooting
There are many things that can go wrong when adding CJK support manually. If something seems not to work, the following checklist might help you.
Obviously, since you run LATEX as a user and not as root, you must allow ordinary users to access the new files. Make sure all folders and files are accessible using the chmod command.
If LATEX writes a without problems, but you cannot view it, it is almost certainly because of some problems in the automatic generation of *.pk fonts. They are supposed to be generated on the fly when viewing a file, but this might fail for a number of reasons: double-check ttfonts.map for your custom line first. However, it might happen that your ttf2pk command, which is usually invoked by the viewer, has been compiled without support for the kpathsea libraries. If this is the case, ttf2pk --version will make no mention of kpathsea. As support for these libraries is necessary, you might have to find a new package, or recompile FreeType 1 yourself.

How do I input CJK in Unicode?
There are a number of different input engines, and the choice can depend also on personal preference. The author uses Skim, a port to of the Scim engine. Refer to your distribution's documentation to learn how to install these programs. Configuration of such programs can be tricky too, in the case of Skim you will have to define an environment variable XMODIFIERS="@im=SCIM" before starting X.