Lack of proficiency in a South Asian language was the one cultural trait, whose loss parents believed to be the most regrettable.
(Lack of proficiency in a South Asian language; was; the one cultural trait, whose loss parents believed to be the most regrettable) (the one cultural trait; to be; the most regrettable)
Lack of proficiency in a South Asian language the one cultural trait
was to be
the one cultural trait, whose loss parents believed to be the most regrettable
the most regrettable