I have always understood this to be an affectation, and I don't perceive that it has become any less acceptable as such. It raises no eyebrows to refer to, to pick some examples off the top of my head, Frau Merkel, Monsieur Macron, Signor Berlusconi. Neither is this limited to English, to wit, Señor Messi, Mister Trump.
Using the foreign title or honorific is not intended to respect the culture or nationality of that individual, so much as simply to associate the individual with it. This may be done to add color to the writing, or to ascribe a certain cosmopolitanism to the author, but at least as often it emphasizes the alien, othered, exotic nature of the individual in question relative to the primary audience.
With phrases of such low prevalence as Herr Hitler or M. Mitterand, the Ngram is of questionable value, but the same tool will find counter-examples where the "native" title is not used, and some other title has been popularized:
If their use has declined in recent years, I would argue that it follows a general decline in use of courtesy titles, which to modern sensibilities may seem superfluous or overly formal. For example, my edition of the AP Stylebook (2013) is firmly against the use of basic courtesy titles (those that do not indicate a professional or academic rank):
Refer to both men and women by first and last name, without courtesy titles, on first reference: Susan Smith or Robert Smith. Refer to both men and women by last name, without courtesy titles, in subsequent reference. Use the courtesy titles Mr., Miss, Ms. or Mrs. only in direct quotations or after first reference when a woman specifically requests it: for example, where a woman prefers to be known as Mrs. Smith or Ms. Smith.
When it is necessary to distinguish between two people who use the same last name, as in married couples or brothers and sisters, use the first and last name, without courtesy title.
In cases where a person's gender is not clear from the first name or from the story's context, indicate the gender by using he or she in subsequent reference.