I am new to Joomla!. My question is:
I installed JoomFish alpha version successfully. If Joomla! 2.5 has multilingual support then why we should go for JoomFish?
What is main difference between JoomFish and Joomla! 2.5 multilingual that will prompt people to go for JoomFish?
JoomFish isn't yet available for Joomla! 2.5, but the answer to the question is:
Joomla! 2.5 does indeed allow multi-lingual support in a native form, but you have to duplicate everything (every menu, create a homepage for each language, etc). The main problem in this approach (for me) is that if you plan to have a site with the same structure for each language, and if you make several changes to the main language (adding more menus, for instance), you will have to make those changes to every language!
What JoomFish does is use the same structure for all elements in the site (menus, articles, sections, etc). It is very good for a site with the same structure for every language. For example: if you hide one menu, it is hidden in every language.
So, in conclusion:
If you plan to have a multi-language site with custom/different structures for each language, then I recommend using the native multi-language technique of Joomla! 2.5;
If you plan to have the same structure for each language and especially if that structure is very big, I recommend using (when/if it is available) JoomFish.
A new extension is behing developed, very simmilar to JoomFish, it's called FaLang: http://extensions.joomla.org/extensions/languages/multi-lingual-content/18210