Look into the Wikiprep/interwiki.pm file.
There's a list of recognized interwiki link prefixes where you can add prefixes for links to other languages (e.g. "De", "Fr", ...).
If you want all of them to be recognized, you can also modify $interwikiRegex to match all two-character codes.
Links to other languages will then appear in interwiki directory (old format) or under <interwiki> tags in gum.xml file (new format).