French constraints in a smaller-Bismarck scenario

How would France respond to a 35,000-ton Bismarck with 13-inch guns?

Probably with much the same construction as historically, since French infrastructure constrained the available response.

Thirteen-inch or 15-inch guns are both considered as possible French responses. The more decisive constraint is the infrastructure available to build the ships. Although foreign construction could provide a public explanation for a program, the French still had to work within those physical limits. The expected result is therefore broadly similar to their historical construction.