This is an area of code I'm planning changes to very soon,
so let me jump in before anyone else explains the way it works
now, which won't help you in a week or two.
The way it's going to work soon is this: the first word
appearing before a : character in a link [[like:this]] will
determine how that link is rendered in the final document.
There are several sets of such words that have different
functions. For example, two-letter ones are reserved for
language links, so that [[pl:xxx]] will create a link to
the article "xxx" on the Polish wiki. Recognized URL methods
are another set: [[http:xxx]], [[ftp:xxx]], etc., will make
direct links to those URLs. Another set is the wiki database
namespaces like "talk", "user", and "wikipedia". Those will
create links to wikipedia pages under those name spaces. Each
of these namespaces has a corresponding talk page--"talk"
links to the talk page of the main article; "user talk" to the
talk page of the User page; "wikipedia talk" to the talk page
of the "wikipedia" space article. This correspondence is used
for the "talk page/subject page" link in the sidebar. Coming
next week sometime, the "file" and "file talk" namespaces will
point to the image description pages and their corresponding
discussion pages (now confused with "image", which causes
problems).
Uploaded files are stored in a database independent of wiki pages.
The special-case prefixes "image:" and "media:" access these.
"Image:xxx" means "look for the file 'xxx' among the uploaded
files, and produce an HTML IMG tag". "media:xxx" means "look
for file 'xxx' among the uploads, and produce a regular A HREF
tag". "File:" and the corresponding "File talk:" will be regular
wiki page namespaces that link to a page with special association
with an uploaded file of the same name (so the page will
automatically show its upload history, what pages use the file,
etc.), but will otherwise be a normal editable wiki page.
Combining interwiki prefixes and article namespaces will work
in that order; so [[de:benutzer:Magnus Manske]] will make a link
to Magnus's user page on the German wiki. Image links will need
to come /before/ any others, so to create an IMG tag referencing
an image on the German wiki, you would use [[image:de:xxx]], and
the German wiki would use [[bild:en:xxx]] to link here.
[[image:http:xxx]] will NOT work; you will no longer be able to
inline-render external images. Alternately, I could make it
work for a specifically allowed subset of external sites.
So you will need to translate the namespaces like "user",
"wikipedia", and "file", and their corresponging talk namespaces,
and also the special prefixes "image" and "media". The two-
letter interwiki codes are international, as are "HTTP" and the
other URL schemes.