library(stringr)
x <- "[('one', 'CARDINAL'), ('Castro', 'PERSON'), ('Latin America', 'LOC'), ('Somoza', 'PERSON')]"
str_extract_all(x, "\\([^()]*'LOC'[^()]*\\)")
# => [1] "('Latin America', 'LOC')"
str_extract_all(x, "[^']+(?=',\\s*'LOC'\\))")
# => [1] "Latin America"