This is now defined in the Language [lang].xml file in the Language metadata settings. If you are having locale problems such as dates do not appear in your language for example, you might want to check/edit the entries in the locale tag. Note that multiple locale strings can be set and the host will usually accept the first one recognised.

Joomla! runs on the PHP pre-processor. PHP comes in many flavours, for a lot of operating systems. Beside PHP you will need a Web server. Joomla! is optimized for the Apache Web server, but it can run on different Web servers like Microsoft IIS it just requires additional configuration of PHP and MySQL. Joomla! also depends on a database, for this currently you can only use MySQL.

Many people know from their own experience that it's not easy to install an Apache Web server and it gets harder if you want to add MySQL, PHP and Perl. XAMPP, WAMP, and MAMP are easy to install distributions containing Apache, MySQL, PHP and Perl for the Windows, Mac OSX and Linux operating systems. These packages are for localhost installations on non-public servers only.
The minimum version requirements are:

• Apache 1.x or 2.x
• PHP 4.3 or up
• MySQL 3.23 or up

For the latest minimum requirements details, see Joomla! Technical Requirements.

No you don't. Versions of MySQL lower than 4.1 do not have built in UTF-8 support. However, Joomla! 1.5 has made provisions for backward compatibility and is able to use UTF-8 on older databases. Let the installer take care of all the settings and there is no need to make any changes to the database (charset, collation, or any other).

Well you are reading it right now! This depends on what you want to achieve. If you are new to Joomla! and have no clue how it all fits together, just install the sample data. If you don't like the English sample data because you - for instance - speak Chinese, then leave it out.

You indeed can change the Menu Item's Type to whatever you want, even after they have been created.

If, for instance, you want to change the Blog Section of a Menu link, go to the Control Panel->Menus Menu->[menuname]->Menu Item Manager and edit the Menu Item. Select the Change Type button and choose the new style of Menu Item Type from the available list. Thereafter, alter the Details and Parameters to reconfigure the display for the new selection as you require it.

A lot of different languages are available for the Back-end, but by default this language may not be installed. If you want a translated Back-end, get your language pack and install it using the Extension Installer. After this, go to the Extensions Menu, select Language Manager and make your language the default one. Your Back-end will be translated immediately.

Users who have access rights to the Back-end may choose the language they prefer in their Personal Details parameters. This is of also true for the Front-end language.

A good place to find where to download your languages and localised versions of Joomla! is Translations for Joomla! on JED.