I feel like I can join in on this even though all my ponies were in English, because people online use the US set names mostly, and ours aren't always the same.
So as follows, US name and then UK name:
Twice as Fancy Pony - Party Pony
Big Brother Pony - Adventure Boy Pony
Glow & Show Pony - Glowing Magic Pony
Precious Pocket Pony - Pocket Friends Pony
Watercolor Baby Sea Pony - Colour Change Baby Sea Pony
Teeny Tiny Ponies - Teeny Weeny Ponies
The official name in the UK for year 3 Earth Ponies is also "Groom and Style Pony", and "Play and Care Set" is also a UK set name for the early baby ponies.
And the formal name for the Glittering Sweetheart Sister Ponies here is just Glittering Pony, but that one hardly counts.
I always love the French name for the flutters. I like the idea of them as dragonflies.