When I parse the Filipino term 'mangmang', the first English equivalent that pops into my head is 'ignorant'—but that's just the tip of the iceberg.
There are layers here: at its core, 'mangmang' can mean lacking knowledge or education, so synonyms like 'uninformed', 'uneducated', 'illiterate', or 'untutored' fit well. If the word is being used more pejoratively to call someone dull or slow to understand, then 'dense', 'obtuse', 'slow-witted', or 'simple-minded' capture that shade. For a softer, less insulting tone you might use 'uninformed', 'unfamiliar', or 'naive'.
Context matters a lot—'mangmang' can be descriptive (lack of schooling) or accusatory (willful ignorance). In formal writing I’d lean toward 'uninformed' or 'lacking knowledge'; in casual speech people often use 'stupid' or 'dumb', though those are harsher. Personally, I try to pick the word that fits the intent: is it about education, experience, or attitude? That tiny choice changes everything.