I would like to develop on the redundancy idea. I think the message should be not only redundant, but it's should be a sort of a physical RAID - a cross-referenced repeatable array of data.
As for the storage medium, I do but think digital storage will cut it for 50 000 years. Contemporary digital storage is actually quite unreliable and decays quickly. Without constant attention and efforts to transfer it to new media the message will be lost quickly. Some answers propose disks and plates from exotic materials - I do not think that is going to cut it too. The storage medium should not be valuable by itself, nothing that future humans would like to melt, turn to jewelry or tools, etc. Rather it should be something comparatively worthless of itself, but durable and chemically stable. Fired clay tablets seem too be a very good variant, carvings on the cavern walls sound good too.
So, returning to the topic of redundancy, I will speak on the example of the tablets. Each tablet is a separate page of your message, and should be explicitly marked as such. So that if a tablet in one storage facility is damaged, it should be easy to understand, which one should be substituted instead. Also, perhaps quite obviously, the tablets with the same parts of the message should be absolutely identical - the same number of rows, the same line separation, absolutely the same shape of letters and drawings (if there are any).
Obviously, each storage facility should also have maps pointing to all the other facilities.
As for the language problem - the Rosetta Stone approach seems the only one to go. Most likely, the tablet should have text in several languages running in parallel. I do not think whether there is any optimal way to select the languages - there is absolutely no way to predict the evolution of languages for time period that big. I would choose languages with drastically different writing systems, just to avoid confusion between languages.
If possible, if the message itself is comparatively short, but decent amount of space can be devoted to storage, the message can include a sort of ABC - tablets with a single drawing and corresponding words in all the language for the most basic and important concepts.
