It's not a defined conversion, it's the literal, internationally ratified definition of what those units are. Or maybe "redefinition" ought to be the word there; prior to that definition there were several very similar, roughly equal but ultimately not internationally standardised units in use. And since they were redefined in terms of SI units, they're technically SI.

This is one of those "tomatoes are technically fruit, but no-one with good sense would put them in a fruit salad" situations.

It's not a recognised translation- it's the definition.

The other way around maybe, that is, an English word becoming technically foreign because we decide that we are going to write its definition in a different language in the dictionary.

It wouldn't make sense to do that though, which kind of breaks the analogy. Unless you count words borrowed wholesale because we didn't have that word, and those definitions were written in a foreign language first.

As it is "one pound" now translates exactly to "nought point four five three five nine two three seven kilograms" where it didn't before 1959. "kilogram" is one of those foreign borrowings.