"He has the least sweet apples."
Translation:El are cele mai puțin dulci mere.
When using the relative superlative, you would usually place the adjective before the noun:
He has the least sweet apples. - El are cele mai puțin dulci mere.
He has the sweetest apples. - El are cele mai dulci mere.
You could say "El are merele cele mai puțin dulci." (note the definite article that pops up on the noun), but it's less common.