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.

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.

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.

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).