A UK style guide's advice
The Oxford Guide to Style (2002) addresses this problem as follows:
Occasionally it may be necessary to indicate italics in text that is already italicized, especially in titles or foreign text. In this instance the opposite font—roman type—is chosen:
[Relevant example:] La Physiognomie arabe et le Kitâb al-Firâsa de Fakhr al-Dîn ...