Are heteronyms unique to English and why do they exist?

We have many heteronyms in our language because English spelling is so illogical. Other languages do have heteronyms. For example, googling "French heteronyms" turns up the pair

des fils: /fil/ some thread,
un fils: /fis/ a son,

which only works because the French spelling of un fils is irregular.

French has d'eux (of them), and deux (two). Borderline case, admittedly, but famous in legal circles for the will that left "a chacun d'eux/deux mille francs" leaving the lawyers to argue whether it was an apostrophe, meaning each heir got a thousand, or a speck, meaning each got two thousand. And that is why old-fashioned English courts still do not use punctuation.

(That's the story I heard, and I'm sticking to it)

Oh, all right then: there are far more possible meanings than combinations of sounds or letters, so some combinations get used more than once. English, having a larger vocabulary than most languages, has more of such 'multiple-uses' though most are either written the same with different pronunciation (homographs) or vice versa (homophones).