I agree for something that's clearly customizing the UI of the whole site, but if it looks like you're choosing a language only for the currently displayed snippet then I think that wouldn't qualify?
I think it still qualifies - in the guidance doc (and a related doc focused on tracking cookies) the focus is on the purpose of the cookie - the intention is to avoid tracking the user - a session cookie that is specifically for remembering a choice a user made in relation to the UI passes the test of a) being necessary to enable a piece of functionality and b) being set as a result of an explicit action on the part of the user in clicking on the particular language tab.
I think the place where it gets tricky is whether you're meeting the "strictly necessary in order to provide an information society service explicitly requested by the subscriber or user" standard. If the control looks like it only affects the current box, then I don't think storing the choice to apply beyond the current page meets that standard.