Leaving aside the problem of actually deciphering the message in 50,000 years, there is one storage medium that is almost guaranteed to make it.
Say it with diamonds.
Most natural diamonds have ages between 1 billion and 3.5 billion years.
Diamond is possibly the most chemically and physically stable material we know. Most natural diamonds were created before humans existed, and will likely be around long after we're gone.
Sub-surface laser engraving allows the creation of internal engraving in a diamond. This is hard, but there are companies that do it today.
Artificial diamonds are cheap. (Really cheap). Thus:
- Get yourself a truckload of artificial diamonds
- Engrave the message below the surface with sub-surface laser engraving.
- Sprinkle liberally
The message is very likely to survive in extremely inhospitable conditions. The only issue is high-temperature fire, which will cause diamonds to burn. this should be covered with redundancy and choosing good locations for your diamond messages.
Diamond's desirability to humans, especially when cut and polished, is likely to remain for a very long time. Human fascination with gems has existed as long as we have been able to produce them, I think it's fair to say that it will continue long into our future. Making the message intrinsically desirable, precious and worth preserving will help it navigate the next 50000 years of human history. Leaving large piles of cut and polished gems in out of the way places around the world will likely ensure that some caches remain in 50,000 years.
You can always use Quartz instead, as is being done right now but really, where is your sense of poetry?
Build a stellar clock
I'd also recommend placing the message in multiple satellites with different decaying orbits. Ensure that every 25 years or so, a new satellite makes it to the surface, with a copy of the message, on the aforementioned diamonds, and instructions to await a second message. That would require a ~2000 satellite constellation, which is a reasonable number.
Having a regular event, that occurs within human lifetimes, and leads to the message being (re)discovered, would also ensure that the problem of language is solved, as it is likely that the new message would be translated every arrival. This narrative of a message from the sky being delivered every 25 years would also likely survive extremes of cultural changes. Such cosmological events as comets or asteroids have been remarked on since we were able to remark on them at all. It's a bonus point that the satellites will likely be visible to the eye and telescope, especially if deliberately coated in reflective material, which means that their orbits can be tracked and their decay predicted, even if we have to re-invent orbital mechanics to do it.
Increase the delivery rate and redundancy to improve the likelihood the message will persist. It's quite conceivable that you could have a yearly programmed orbit decay for 50,000 satellites. (compare this number to the total number of satellites we've sent up so far which is ~8900).
The stellar clock mechanism has the advantage of being very resilient to cultural changes, while at the same time creating an intrinsic cultural focus for any civilisation. Think Halley's comet, except it happens more often and something falls to earth in a pillar of flame. I doubt many civilisations would miss the hint that something is going on.
Risks are that gravitational disturbances would cause the clock to go off sync, although it's unlikely to change the mechanism entirely, it just means that the periods may be perturbed.
Bonus if you call your constellation "Lucy" For one massive, 50,000 years Beatles song joke.
Reality check: This assumes that the Lyapunov time of earth-orbiting is greater than 50000 years, which may not be the case at all, although it strongly depends on the orbit specifics, in which case it's a pretty idea but not really viable.
Build a foundation
Isaac Asimov's Foundation Series is predicated on this exact problem, and how Harry Seldon solves it. I'll let Asimov explain this more eloquently than I could in his books :)