Planet Sherlock (or Sherlock, the Universe and Everything)
It’s an ordinary day at 221B apart from the fact that Sherlock is speaking in French. What John doesn’t know is that Sherlock has an ulterior motive. What Sherlock doesn’t know is that while John may not speak French, he’s almost fluent in Sherlock.
The original, the best, and soon to be the first of many. Now revised. Sherlock's dialogue is actually in French; optional annotated translations within.