It's raining in Paris.

The French means - you are looking for a mother in law. - presumably said to someone with a petticoat longer than her dress.

Charlie's dead?

Vive l' empereur! Your flies are undone

Your underskirt is on show

We used to say 'its snowing in Paris'.
If a man's flies were inadvertently open my Dad's generation would say ' you're wearing your medals I see'. Bet there are many variations on that.

Since they did away with buttons and put zips in mens trousers life has been a little more dreary hasn't it?

I have always used that expression - your medals are showing - when DH has inadvertently failed to check his trouserage. I once had occasion to use it to an officer I worked for - the most immaculately dressed man I have ever met - who I passed in a corridor on his way back from a lunch at the Cavalry Club with a bit of his shirt peeking out of his flies. His reaction was not to bat an eyelid but just to tip his bowler look straight ahead and mutter "noted Corporal!"

Flying without a licence!

Charlie's dead and flying low

'Noted corporal'! A cool customer indeed!

And when you had your period, Aunt Jane came to call .