> it is traditionally typeset as italic (so simply marking it with a <span> is losing information).
"traditionally typeset as" is not information about the content, its information about historical typographical practices.
Ideally, over time, the actual semantics, rather than merely the traditional presentation, of the things where the "B" or "I" tags might be the currently least-bad standard tags would be defined in HTML. Though <span lang="fr"> is already semantically adequate for the specific case you present because the "foreign" is implicit in the span having a different lang attribute than the surrounding content, and the "i" is just an irrelevancy about traditional presentation of foreign-language content.
"traditionally typeset as" is not information about the content, its information about historical typographical practices.
Ideally, over time, the actual semantics, rather than merely the traditional presentation, of the things where the "B" or "I" tags might be the currently least-bad standard tags would be defined in HTML. Though <span lang="fr"> is already semantically adequate for the specific case you present because the "foreign" is implicit in the span having a different lang attribute than the surrounding content, and the "i" is just an irrelevancy about traditional presentation of foreign-language content.