When translating a page, the linked pages, if they exist in the target language, are replaced by a link to the equivalent page in the source language. If they do not exist, it is possible to mark them 'As missing', after selecting them. In this case, they are turned red for the target language.
But it is a pity that there is no option to generate a [[Template:Interlanguage_link]] tag that would allow having the red link for the target language, as well as the link to the page of the source language.
Indeed, a red link does not say if the page does not exist in another language while the ill links do. On the one hand it allows navigating in the page in the source language (if it is English this page is still readable by many readers), on the other hand it allows to indicate to potential translators in which language this page already exists.
I constantly come across French pages with red links, and most of the time I realize when I go to the English wikipedia that this page exists in English.
I think this feature would be extremely useful to integrate into the translation tool.
NB : I see that my remark applies more to Wikipedia pages translation, I don't know if this is the right place...
Thanks