If you define a fruit as, For example, A reproductive portion of a flowering plant, Then by that definition a cucumber is a fruit. If you use a different definition, Then perhaps a cucumber is not a fruit, By that definition. I am neither a botanist nor a linguist, So my personal opinion on this subject should carry little weight.
They are fruits. Some fruits like tomato's and avocados aren't sweet so fruits don't have to be sweet. Some stores e v e n p u t t h e c u c u m b e r on the fruit shelves, Though these are opinions and I might be wrong.