By Diphthongs, Expressions

Two vowel letters are something you can see with your eyes in a word like soul. Two vowel sounds is something you can hear with your ears only, not see in print: gave. In the last example there is only one written vowel, a. There may be languages in which diphthongs are pronounced even when they...

Definitions from the Oxford Advanced Learner's Dictionary:diphthong:(phonetics) a combination of two vowel sounds or vowel lettersmonophthong:a speech sound that consists of only one vowel soundtriphthong:a combination of three vowel sounds or vowel lettersThe above definitions use the...

If English-speakers learnt how to pronounce the Latin expression per se, they wouldn't want to transcribe it as per say. The e of se is not a diphthong. In no English word is a solitary e pronounced as a diphthong, so why imagine it is so in other languages? Try removing the t from set.