Is it permissible to work for an establishment that produces soft drinks, which belongs to a group that includes an establishment producing alcoholic beverages, provided that my role is to monitor the quality of the soft drinks?

If the institution where the internship will take place is separate from the institution that produces alcoholic beverages, such that there will be no witnesses to forbidden acts in your presence, nor assistance in them, then there is no harm in that. However, if the internship necessitates mixing with non-mahram men or being alone with one of them, then it is not permissible to join unless there is an extreme necessity, while limiting oneself to the extent of the necessity and observing the controls of Sharia.