The term for "only begotten" has been controversial. It is the Greek word
monogenes (μονογενής), as is well known. Modern English translations render with "only" or "unique" or something similar. If I had to, I would go with "unique"; I feel "only" is too weak. Jesus is uniquely the "only begotten" because He is the only Son of God who was physically born. We are sons of God also, but our birth as children of God is spiritual, not physical.
There was quite the furor when the RSV came out in 1952 with simply "only" for
monogenes. That was the first version to be translated by liberals, and the immediate charge was that "only son" was a liberal rendering. However, in recent years some scholars have claimed that "only" is okay, based on the NT usage, and that the etymology of the word is not
mono=only and
genes=begotten. I won't try to set forth all the arguments here, but I'll just say we went traditional in our Japanese NT.