There's an inconsistency because singular and plural phrases were created for other minor areas (e.g., Star Conversation and Star Conversations). However, I feel like this is as designed; as such, I'm creating it here rather than XenForo Bug Reports.
When there's only one reply, "one replies" just sounds really weird. I don't know what template it's called in, but its phrase is x_replies, which contains:
I suggest creating an additional x_reply phrase, which would contain: