For what it's worth, the "and" is mandatory in my language and we manage to disambiguate 100 2/3 from 102/3 by putting a pause between either "hundred" and "and" or "two" and "thirds".
100 2/3 - one hundred ... and two thirds
102/3 - one hundred and two ... thirds
Alternatively, use "plus" instead of "and" in the first case.
Surely that comes from the same language structure which means you wouldn't say beans and eggs and tomatoes and potatoes, you'd say beans, eggs, tomatoes and potatoes e.g. in a list of "and" items you only say and between the penultimate and final items?
100 2/3 - one hundred ... and two thirds
102/3 - one hundred and two ... thirds
Alternatively, use "plus" instead of "and" in the first case.