The best hack for related languages no one uses.

It is shared phonetic mapping. Languages apart of the same family share many cognates, so why do we learn each language as if we cannot apply the vocabulary gained in one immediately to the other? Two primary reasons: graphing and sound shifts. Take the words “Heart”, “Καρδια”, and “Cor”— First is English, the second Greek, and. the third Spanish. If you did not know they were cognates, you might imagine these are completely novel formations with little connection— modern ideas built in each language individually, with morphemes added or subtracted from their root proto-indoeuropean words. However, one who understands the phonemic shifts per branch would realize each are different expressions of the same IE word.

Καρδια, romanized: Kardia. Root: kerd-, plus -ia case ending. The greek shares the older English pronunciation “Kheart”— though aspirated K’s transformed to h’s and d’s to t’s. Spanish writes the “k” sound as “c”, and keeps the original IE “e”, uniquely opting for graphing it with “o”.

The takeaway is that with some case, phonetics and understandings of how each language maps morphemes, you can up your vocabulary early using just one of the languages.