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! 1.5 does not provide an upgrade path from earlier versions. Converting an older site to a Joomla! 1.5 site requires creation of a new empty site using Joomla! 1.5 and then populating the new site with the content from the old site. This migration of content is not a one-to-one process and involves conversions and modifications to the content dump.
There are two ways to perform the migration: