Since, the sentence starts talking about the thought process, R will be the next statement as it is talking about the two factors. Q will be the continuing statement as it carries a relative pronoun which connects both the statement. S follows Q as it discusses the need for a language and P discusses why there is a need for the international language.
The correct formation would be, ‘We can think of two factors which suggest the need for an international language often confused in the public mind’.
