There is a formal way. Purchase and install the appropriate language  pack for
SharePoint. Then change the regional settings of the site  to use the new
language. That will change all the prompts. You will still need to translate
all the content manually.

We have already purchased the language  pack and installed.
I am not talking about the Locale, Sortt Order etc. I am talking about the
language in the menus etc.
I couldn't find any option for that in the Regional Settings.
If I cretea a new site  collection, I can specify the language and all the menus
are in the selected language.

I did some more research after my original reply. There is no supported way to
change the language  of a site  after it has been created. The LCID is stored in
a ReadOnly property of the SPWeb object that represents the site. If you have a
language pack installed you could go to the WEB table of the content database
and change the Language field to the numeric LCID value of the other language
pack that you installed. There are 2 problems with this approach:

1) Since you are modifying the database directly it VOIDS your service warranty
with Microsoft. You will no longer be able to contact Microsoft for support.
2) Changing the LCID of the site WILL change the dynamic menus to the alternate
language. It will not change the static values that were set by the language
choice at creation. For example, all the labels of web parts, list names, list
descriptions and list/library URLs will be in the original language and would
need to be edited manually.

The language  pack must be installed prior to creating the site  in the desired
language.
The site is then created from the site template (definition) of the language
selected.
Once built, the locale of the site can be changed but not the language.

You could save the content of the site, delete the site and create in new
selecting the language needed and place the content into the new site.

You may want to read the languages and variations chapter of the Administrator's
Companion.

It would be interesting to see what happens if you save a list or document
library as a template which is a delta from the original definition and then use
the template on a site  where the new definition is in the new language.
I will try this as soon as I can get caught up on some writing, probably early
September.

I also (since I won't touch the database directly) do not know if all the
management pages change languages if you hack the LCID in the table.