I have been reading a bit about passphrases strength and weaknesses. I will take this answer as example.
TTT says, among the other points:
Although there are hundreds of thousands of words in the English language, we (probably) only need to try brute-forcing passphrases using the set of the most common words. We'll assume there are 3000 words in that set.
Leading to an entropy (correct?) of
3000^7 = 2.2 * 10^24
This makes the (generally understandable) assumption that the passphrase will be in English.
My question is: removing that assumption, i.e. that each word is from a different language, and that I do not necessarily need to speak that language to remember how to spell one (or two) word(s) [*], how would one compute the entropy of such a passphrase and estimate if the passphrase strength benefits?
[*] : this is to remove the possible drawback of reducing the languages space by knowing the target, i.e.: "I know they speak only English and Spanish, thus the passphrase can only contain those two languages"