I'm not sure is this possible,for example I have english,french and italian blog,they all use same theme,I just want when visitor is on french site for example to display date,(some strings like search,next page,...) on French,can I do it with theme localization an how,how to setup wordpress multisite to use this theme localization?
Basically after you install the language packs, go to that site and change the language option from the admin area.
There are a few parts to your question:
for dates etc, you should have the wordpress translation files loaded - then dates will be 'localised', so long as the theme or plugin uses http://codex.wordpress.org/Function_Reference/date_i18n (wordpress does, so if it is it's code it should be fine)
For any theme generated strings, then yes your theme must use the __() or _e() translation functions (any half way decent theme should have been written with this in mind) and have the necessary language files.
If no language files exist for the theme, it is easy to create them using a variety of tools (eg : code styling localisation plugin)
See http://codex.wordpress.org/WordPress_in_Your_Language http://svn.automattic.com/wordpress-i18n/ (wp language file respository)
theme developer should know this: http://codex.wordpress.org/I18n_for_WordPress_Developers
There is special plugin for this, called Quick Localization. It is developed by my company.
You can patch any plugin or theme as long as they use
_e() functions properly.