Maybe theoretically Spanish has no diphthongs. But rapid speech definitely sees diphthongs develop as sounds merge together -- a very much Spanish thing.
Sure, but here Spanish orthography is actually representative of the sound, whereas English orthography is a mess (plus English has more than 5 base vowel sounds to begin with).
No matter how you cut it, English's vowel phonology is much more complicated than Spanish or Japanese. One misconception that native English speakers have is they think their language is simple to pronounce, when it really isn't.
https://en.wikipedia.org/wiki/Diphthong#Spanish
Your example would be /kau̯.da/, with the a and u merging into a single syllable diphthong.