They were called 'superimposed titles' to differentiate them from intertitles, which were interpolated between moving picture footage, rather than superimposed. Intertitles were referred to at the time as subtitles, to add to the confusion. Reading the rather plodding explanation below, we should remember that these were viewers used to silent films, where the action had to stop when a title card appeared. The idea of a title running concurrently with the action was a bit revolutionary:
Witnessing a picture with superimposed titles, so-called, is an odd adventure. Superimposed titles are explanatory subtitles printed on scenes in the film, which translate the purport of what is being said in another language. They are being used in Europe on the American pictures. The words spoken in English are those translated into German, French or some other language, depending on what country the picture is shown in, so that its dialogue may be understood by the audience.
I saw the French film, 'David Golder,' with superimposed German titles, and a knowledge of that language made it exceedingly easy to folow. The words spoken in French, which is often a difficult language to understand conversationally, acquired meaning as the picture progressed. It would seem, therefore, that the viewing of pictures with superimposed titles is an aid in acquiring a knowledge of a different tongue. Nor is it as complicated nor as distracting a method of witnessing a picture show as might be imagined. (2 January 1932, p.19)
Harry Baur, as David Golder (Duvivier, 1931) with his horrible daughter Joyce, played by Jackie Monnier
As a bonus extra feature on this blog post, here's a snippet by Harrison Carroli a few months later in the Evening News of Wilkes-Barre, Pennsylvania:
A superimposed title, in case the term leaves you with a blank feeling, is the old-fashioned silent title cut down to a narrow strip and imposed across the bottom of the image on the screen. In Japan it's a perpendicular strip along the side. This title translates or explains the English dialogue, which goes on without interruption.(22 April 1932, p.9)