>> I know that we can configure MSP for several languages, but is there a feature internal to MSP or an add-on that can be purchased that would allow this to be done?
To my best knowledge, no such software exists for Project if what you need is on-the-fly language translation on the task names and notes. I would copy the task names and notes and paste into Excel, and then translate all the text using an online translation service if the machine translation is sufficient. Custom text fields inserted next to the task name and task notes columns can be used to store and display the translated text. We can, of course, hide the columns anytime we want, and it is always safe to keep the original task names and notes in the file. If you need human translation, then the Excel file can be handed over as pdf, to a translation office offering services to your particular business. Otherwise, Project would identify the elements of the mpp file prepared with Spanish version by internal ids and allow us to open the file in English version with no problem on the field names etc… Some minor work may be required with filter names or other customized items in Spanish. It is always better to get a list of customized items from the client.
This is my opinion/suggestion. Let us keep following the discussion as the other MPUG members working in multi-language project environments may offer better solutions.