At times, well-meaning, but ignorant colleagues may provide unsound advice. You may want to smile politely and ignore them.

The Good Samaritan law does not apply in this case. Washington state's Good Samaritan law protects from civil damages anyone who provides first aid or transportation during an emergency without expecting to be paid.

There is a Duty to Rescue law, also known as a "Bad Samaritan" law, which holds people responsible for not helping someone in imminent danger. It appears from your description of the situation that this law does not apply either. 

Beyond that, the issue of whether you may provide interpreting services for another patient is not an ethical one, but a contractual one. If you are required under your contract to provide interpreting services for patient X, you are not contractually obligated to interpret for any additional patients. There is no ethical reason that you cannot do so if you so choose. 

On the other hand, If you are being asked to provide services other than interpretation (e.g. administer medication) this would be beyond the scope of your professional responsibilities and indeed a liability concern, as well as an ethical violation.