I have given another shot at making this blog run in multilingual mode, this time with another plugin (xili), which seems to be better integrated with internals of WordPress. This forced me to fix some the french translation file of the sixteen theme, which was completely broken. Things kind of work now, i.e. the blog’s chrome will render in French for a blog which is French, but there still is one bug: the theme uses the JQuery timeago module, and generating date texts within Javascript means that the PHP localization framework won’t work. There is an update version of timeago, which includes support for localization, I’m not sure to what extent I’m willing to fix the current theme or just switch to better designed one.

Edit: I fixed the theme.