When I try to add wiki links to translations, they are wrapped in <nowiki> tags, which is not what I expect them to.
Content Translation is WISYWIG, like Visual Editor. It goes even further than Visual Editor, because it can auto-adapt links from the source article. To add a link, select some text and use the tools in the sidebar to add it.
Typing [[link in square brackets]] will cause <nowiki> at the moment. In the future we may do the same DWIM trick that Visual Editor uses when typing [[.
I only partially understand this all, because visual editor is completely outside my world and "even beyond" is even beyond it :-) Visual editor usually makes my browser choke or causes other unpleasant effects, etc. Can one translate without all these useless distractions and time consuming bells and whistles just typing ordinary wiki text?
Sorry, no. Using WYSIWYG in the case of Content Translation is not just for making editing easier for people who don't know wiki syntax, but also for easier link adaptation. See the FAQ.
But it's really not bells and whistles: just select the text you want to link and watch the side bar for the link tool.
DWIM = "do what I mean". A lot of people (like you) are used to type "[[" to start a link, so Visual Editor is clever enough to automatically open the link tool when people do it. For people who come from the Google Docs world, it also has the Ctrl-K shortcut, which does the same thing.