Communication Breakdown

If a dignitary from a non-Anglophone nation speaks a language that is not his native tongue, yes, you can introduce grammar mistakes and such. Even an educated man may stumble in a foreign tongue. If you have him communicate in his native tongue and have someone from his own or another non-Anglophone country translate for him, this rule also applies. Translators are people and people make mistakes. But if you have him speak in his own first language and then mention that the English lines we read in the script are the subtitles FOR THE LOVE OF PETE don't introduce any 'foreign speaker' grammar mistakes. It's his native language. Unless the character is uneducated he probably speaks it fine. Unless the person creating the subtitles is a character in the story (ala Monty Python and the Holy Grail) subtitles are pure knowledge. Grammar and word choice mistakes here will make the production company look dumb, not the character.