Ah, the perils of a limited vocabulary. Grammatically inflection does not mean tone, it means that the endings of words (the inflections) change depending on the function of the words in a sentence. Latin and Greek are inflected, Thai and English are not.
Dick, when has anybody ever said "Khun Thai" where "Khun" is the polite participle? I admit it's possible but it's a highly unlikely usage.
What? Why on earth would use such a weird usage. You do know that Khun is the relatively polite form of you, don't you. Khun Farang is not remotely rude. Ai Farang or Ee Farang would be (although they're prefixes, not pronouns), but not Khun Farang. They most likely reason for anybody to use Khun Farang would be because they don't know your name.
BTW, khun does NOT = you, khun is just one of a number of pronouns that can mean you depending on which register of Thai you're speaking in. Others are ยู (although that one's really English), มึง, นาย, or ท่าน. คุณ is the standard polite pronoun in the range of pronouns for you and nobody would use it if they were attempting to be rude.