Besides English, is there any other language where the 2nd person singular converges with the second person plural?
And is there any other language where the informal singular 2nd person converges with the formal one?
Linguistics Stack Exchange is a question and answer site for professional linguists and others with an interest in linguistic research and theory. It only takes a minute to sign up.Sign up to join this community
It happened to Dutch much longer ago than in English. The original Germanic 2nd person singular pronoun is no longer recognised as a Dutch word, unlike "thou" in English which is still recognised as an archaism. The word "du" got replaced by the plural form "gi" (later "gij") in Middle Dutch, as it was polite to use the plural form. This politeness eventually spread, replacing "du" entirely.
"Gij" developed into "jij" in standard Dutch and the old form "gij" is now recognised as an archaism and regarded as the Dutch counterpart of the English "thou". At that time the 2nd person singular and plural were the same, just like the current situation with English "you".
This situation didn't remain stable, however, as people started to say "je lieden" (meaning "you people") for the plural meaning. This developed into the current 2nd person plural, which is "jullie".
The Dutch language also gained a new polite 2nd personal pronoun. People started using "uwe edelheid" ("your nobility") as a formal form of address, which was shortened to "u.e." and subsequently simplified to "u". (Note however that there already was a word "u", which was the oblique case of "gij" and goes all the way back to the time it was still only plural.)
In sum: the Dutch 2nd person singular converged with the 2nd person plural. But then Dutch developed a new plural and a new polite form.
Sources: my personal knowledge and Wikipedia.