A bilingual book is printed in two languages. One language is printed on the left-hand page and the corresponding translated text on the immediate right-hand page. One language is usually 30 percent longer than the other.
To make these books readable, different printers typeset the fonts differently. The easiest way to differentiate is to do it with color. The result is visually stunning. This strategy, however, falls short if using color is not an option. Other than different ink pigments, one can use varied positioning for type area. The eye of the reader will know where to begin reading. This procedure cannot work if there is a requirement to save space.
Another solution is to sit one text on another. This can be the hardest as you must find the same baseline for the two different fonts. Use Serif and Sans versions of the same family. Doing this will maintain similar contrast. Do not use dissimilar weights.
Yet another method is the use of italics. The principal language should be bigger and on top. The secondary language should be in italic. The best method to choose this is to look at similar projects or client booklets.