I would pronounce
Lotte "LOT-uh" as it's a Germanic shortening and that's how it's pronounced there.
Lottie is the English version (LOT-ee).
I love both
Lotte and
Lottie, but I'm not a huge
Charlotte fan. I prefer them as short forms of
Lieselotte or nicknames for slighly different names like
Hannelore.