How many languages are mnemonic seeds currently offered in other than English?
Does the entropy for different languages depend on the number of words the relevant dictionary has available to draw from?
I can answer the second part of your question regarding entropy.
Regarding the 1st part of your question, please understand that understanding the meaning of every English word is not required to use English mnemonic seeds. While adding a few more popular language may make some sense at some point the benefit of additional language support will increase security risks (from bad implementations, etc) and support costs more than the marginal benefit provided.