The default worldlist is the same as the bip39 one but an interesting aspect of the electrum mnemonic algo is that it's not tied to any specific wordlist.
but isn't that true about any other implementation of BIP39? basically the wordlist itself is not important, it might as well be anything because each word only represents a number (index in the list) which is then used to generate the master private key.
other wallets may not have the interface feature for the user to do this but they have the capability of doing so, i believe.
The wordlists are a part of the bip39 standard. They have to update the standard each time they add another language
If you look at check function here or bip39_is_checksum_valid in electrum you will see that the wordlist is used to verify the checksum.
In the case of Electrum's own seed mnemonics the seed version is encoded in the mnemonic and it serves as a checksum and as a way to identify the wallet type. You can see how electrum verifies it's own seeds here (on_restore_seed) and here (is_new_seed). As you can see the dictionary is not being used to verify the seed mnemonic. Only that it hashes to a number with the correct prefix i.e. a valid seed version.
If you want to use a custom wordlist with electrum see here. Note replacing the wordlist with your own will mean that Electrum can't verify bip39 seeds anymore.
