Here are some words translated from artificial language.

'sin co bye' means 'He is good'

'co mot det' means 'They are good'

Which code word stands for 'good; in the artificial language ?

co

'sin co bye' means 'He is good'

'co mot det' means 'They are good'

We can see that good word is common in both of the statements given above.

so the common word for good will be the code for good.

So, the common word in both of the statements is co.