Creating a Multilanguage Site with D5.x

The specific answer to your question is a bit complex, and you probably won't find anyone who is an expert in all available modules, but let me give it a shot.

First, the most "efficient" way to translate the various contributed modules is if you have a .po file in the language you need for that module. If you have this file, then when you install a module, you go to Admin > Localization and add the .po file for that module just like you would add a new core language. (See my documentation page here: http://drupal.org/node/114756) This will translate the strings accompanying that module. Note that often .po files in different languages may not be bundled with the module as they may have been done by independent translators who haven't submitted them to the original author. A search on relevant language sites can sometimes be very fruitful.

If you don't have a .po file, you can still translate these strings from within the Localization screen. Go to Admin > Localization and click Manage strings. Then do an empty search for untranslated strings.

For content (and a bit more), you have your choice of various contributed modules. The Internationalization module or Localizer module are the most widely used. I chose Localizer so that is the one I am most familiar with. In addition to content translation features, the latest version of Localizer provides a translation management screen for "added" strings such as your site name, mission, etc. If you are interested in Localizer, you might find my guide of some help. http://drupal.org/node/114750. For the Internationalization module, the documentation is at http://drupal.org/node/133977. (Note that you should NOT install both.)

I am afraid I am not a coder, so I can't help you specifically with your RTL question, though I have seen multi-lingual sites including RTL done with Localizer. (I suspect the same is true for Internationalization.)

Thanks guys, that was very helpful. I found out that i18n adds language strings (en/fr/de) to the URL, which makes it possible to load different templates depending on URL aliases : http://drupal.org/node/139766
it is also possible to query the global language variable defined in i18n api, and load different stylesheet depending on it. (i'm trying that out)
another question , since D6 comes with better multilingual support and RTL support out of the box, do you think it is easier to try to migrate D5 modules to D6 and make use of the multilingual capabilities in D6 instead of trying to work on D5 ?

Thanks again for the great advice

Excellent ! thats exactly what I was looking for !

Thank you for pointing that out