Are there more words in word lists in some languages than others? If so how would my language choice impact the entropy of my randomly created mnemonic seed?
The seed is a reversible transformation of the private keys. Therefore, its entropy is equal to the private key's entropy, which in turn is whatever entropy was used to create it, assuming none was lost in the process.
All word lists for supported languages do this same job: to perform a reversible transformation, and each private key set can be converted to any of those languages. So a seed has the same entropy in each of those languages.
I will answer your second question first:
Yes, a longer word list would lead to increased entropy and a shorter list would lead to decreased entropy.
Monero avoids this problem by relying on an Electrum word list that is exactly 1626 words for each language. Because of this, the entropy of mnemonic seeds created with simplewallet for each language below will be the same.
0 : English
1 : Spanish
2 : German
3 : Italian
4 : Portuguese
5 : Russian
6 : Japanese