Is it possible to "translate" a mnemonic phrase in English into Spanish (bip39) so that they lead to the same private key? For example, if we match each word in the English dictionary with the corresponding word in the Spanish dictionary (comparing entropy)?
The words/meaning of the words doesn't really matter. What matters is their order.
Mnemonic word is basically word representations of a long number. Each world represents a number between 0 and 2047.
This is how a mnemonic phrase is generated
The mnemonic must encode entropy in a multiple of 32 bits. With more entropy security is improved but the sentence length increases. We refer to the initial entropy length as ENT. The allowed size of ENT is 128-256 bits.
First, an initial entropy of ENT bits is generated. A checksum is generated by taking the first ENT / 32 bits of its SHA256 hash. This checksum is appended to the end of the initial entropy.
Next, these concatenated bits are split into groups of 11 bits, each encoding a number from 0-2047, serving as an index into a wordlist. Finally, we convert these numbers into words and use the joined words as a mnemonic sentence.
For example, the first mnemonic word for English is abandon, and the first mnemonic word for Spanish is ábaco. They represent the same number but has totally different meaning.
BUT, you can have Spanish Mnemonic words though.
You can see the equivalent word in the Spanish dictionary using the Ian Coleman BIP39 tool. First you have to select the number of words for your seed. Then you have to enter the seed in English, and finally click on the Mnemonic Language in which you want to see the equivalent. https://iancoleman.io/bip39/