Adding support for language written in a Right-To-Left (RTL) direction is easy.

There are two ways to do that:

• By creating a fully mirror of your style.css file named style-rtl.css
• By overwriting all the horizontal positioning attributes of your CSS stylesheet in a separate stylesheet file named rtl.css.

Method 1: style-rtl.css

• Use an automated RTL css creator such as RTLCSS or CSSJanus to create an rtl version of your style.css file. Save the result as style-rtl.css and place in the same location.
• Use wp_enqueue_style to enqueue your style.css file in the theme. This won't work if the file is simply embedded in the header file
• Use wp_style_add_data to set the rtl property to replace for your stylesheet.

Example:

wp_enqueue_style( 'themeslug-style', get_stylesheet_uri() );
wp_style_add_data( 'themeslug-style', 'rtl', 'replace' );

Notes

• Both of the tools mentioned have developer friendly tools to automate the creation of RTL stylesheets.

Method 2: rtl.css

• Start with your main theme stylesheet (usually style.css).
• Save this file as rtl.css
• Add the following attributes to the body selector:

  direction: rtl;
  unicode-bidi: embed;

• One by one, go over the CSS selectors and do the following: Remove any irrelevant attributes such as font styling attributes, colors, vertical positioning, width and height etc. Change the value (from right to left and vice-versa) of the following attributes:
  • text-direction
  • float
  • clear

   text-align: left;
   float: right;
   clear: left;

becomes

  text-align: right;
  float: left;
  clear: right;

• Add RTL versions of relevant images.

Some images are clearly suited only for one direction (arrows for example). Create a horizontally flipped version of those images.

• Mirror the following attributes, and zero the original
  • margin
  • padding
  • borders
  • background-position
  • right/left positioning

.commentslink{
	background:url("./images/comments.gif") no-repeat 0 3px;
	padding-left:15px;
        margin: 2px 4px 0px 12px;
        left: 10px;
}

becomes

.commentslink{
	background:url("./images/comments-rtl.gif") no-repeat 100% 3px;
	padding-left:0;
        padding-right:15px;
        margin: 2px 12px 0px 4px;
        left:auto;
        right:10px;
}

For buttons that have hidden text using text-indent, you need to change its value from negative to positive:

.image-button{
	text-indent:-99999px;
}

becomes

.image-button{
	text-indent:99999px;
}

Tools

The RTL Tester plugin allows you to easily switch the text direction of the site: http://wordpress.org/extend/plugins/rtl-tester/

P.S. This plugin allows to VIEW ONLY RTL. i.e. How would your site look like when its Text Direction is changed & this change isn't viewed by the viewers, (unless the whole CSS is changed).

Special Cases

• You'll need to manually adjust pixel positioned backgrounds. If the original value is '0' you can change it to 100%
• Positioning: remember to assign the 'auto' value to the original selector
• Some fields should be Left To Right direction in both LTR and RTL languages, For example : email, telephones, zip codes, codes, URLs and any input contains English letters and numbers, So it is better to assign a unique id or class to each input fields, which you will be able to make them LTR direction in CSS files

Example:

<input type="text" class="email-address1 email" id="email-address1" />
<input type="text" class="tel-number1 tel" id="tel-number1" />
<input type="text" class="zip1 zip" id="zip1" />
<input type="text" class="url-address1 url" id="url-address1" />

Using class:

/* These fields should be LTR in a RTL language direction */
.email-address1,
.email,
.tel-number1,
.tel,
.zip1,
.url-address1,
.url,
...
{
     direction: ltr;
}

Writing a Post in RTL Language

WordPress default visual text editor doesn't support writing in RTL languages like Hebrew, Persian and Arabic. Therefore, if you're willing to write a post in one of those languages, you should either :

• Reinstall / Change your WordPress version to be in that language, which fixes this issue with allowing you as a default to write in RTL.
• Use this plugin: http://wordpress.org/plugins/wp-rtl/ which adds two buttons to the TinyMCE editor (if you have it) to enable changing text direction from Left to Right (LTR) and Right to Left (RTL).