You are making a(n incorrect) assumption that "definiteness" is a universal category that is encoded consistently.
The definite article, even in languages with one, can vary significantly in its use! So while a Chinese or Russian learner of English has a lot of trouble with articles, a Spanish learner of English will also have a lot of trouble with certain uses of articles because they aren't like in Spanish.
As MalFet said, very roughly, a general equivalent to English "a" is 'one' and a general equivalent to English "the" is 'this' or 'that', but in fact these aren't really even encoding definiteness strictly. 'one' implies non-uniqueness, while 'this' or 'that' is a deictic word, pointing to a shared concept (similar to definiteness but only part of the puzzle).
If you want to understand definiteness, you should do some reading and consider specificity vs. uniqueness, among other things.
Regardless, many languages simply do not encode this information regularly in the grammar. So you should not assume you will find either or both, but you might!
I have seen that some Dravidian languages do seem to have indefinite articles but not definite articles, while Arabic only has definite articles [with a particular very formal ending used for indefiniteness only in the most formal use of formal Arabic, not in the spoken varieties].