1. I installed with my own language, but the Back-end is still in English
(FAQs/Languages)

... 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 ...

2. How do I localise Joomla! to my language?
(FAQs/Languages)

General In Joomla! 1.5 all User interfaces can be localised. This includes the installation, the Back-end Control Panel and the Front-end Site. The core release of Joomla! 1.5 is shipped with multiple ...