Given that the Celestian alphabet has been in Wizard101 for a few years now, the message on the concept map has already been fully translated. There are multiple blogs, websites, and forum posts who have posted the translated message (and they all have gotten the same answer), so I won't be linking any specific sources. However, a person in the Wizard101 Central forums provided a very helpful guide to the Celestian alphabet a few years back, so here's the link to that post.
So, using a guide such as this, you can translate this message:
And it reads:
EAST OF THE BLUE SUN
WHEN THE STARS ALIGN THE MOON IS KEY
From this message, we know a few things:
Marco Pollo knows how to read and write in Celestian
Cardinal directions exist in the spiral
A diagram of the phases of the moon (pictured below) appears right next to the message, and the mention of the moon, sun, and stars suggest that celestial (no pun intended) bodies are important in locating the stormgate to El Dorado
This still leaves us with a few questions, however:
Are there canonically only one sun and one moon in the spiral? Or does each world have its own moon and sun?
Is the "blue sun" in the message actually referencing a literal sun, or is it a metaphor for a world?
I personally think that the "blue sun" is meant to represent Valencia for a couple of reasons. First of all, on the concept map (shown below), Valencia is colored blue. Now of course, Valencia isn't the only blue world on the concept map- there's actually quite a few of them. But what makes Valencia stand out the most is the world's sheer size compared to all of the other worlds of the spiral, which brings us to my next point: Valencia acts like the sun of the spiral.
Most of my points for this argument, including the picture down below on the left, actually come from a Pirate101 blog known as the Adventures of the Spiral, so I will link their very helpful findings here. As we can see down below, Valencia is drawn bigger than any other world on the map and presumably in the entirety of the spiral (except for El Dorado). This may be due to Marco Pollo trying to emphasize Valencia's importance and prestige, however, the world may actually have the largest mass given that it has the most stormgates out of any other world. This fact is easily seen within the game: in the Calabria skyway alone, there are 4 different stormgates that lead to other worlds, and we haven't even explored all of the Valencian skyways yet.
Not only is Valencia massive, but it is continuously believed to be at the center of the spiral. In the image from the Adventures of the Spiral blog above, it can be seen that the Valencians depict their world to be at the center of the spiral and surrounded by all of the other in-game worlds. In the story of the game, Valencia is also recognized as the center for trade and the arts, so the world being at "the center" can be metaphorical as well. Nonetheless, I think that the most compelling argument for Valencia being the "blue sun" is the fact that in the concept map, Valencia is in a central position and has multiple worlds orbiting around it, just like a sun in a solar system.
Therefore, I think that Valencia could very well be the "blue sun" that Marco Pollo describes in the Celestian Message.
A diagram of the phases of the moon and its cycle appear to the left of the Celestian Message
A diagram of what looks like a planetary alignment appears right above the Celestian Message
I believe that "the stars" mentioned in the message are a reference to certain planets; as I mentioned before, I think that this map depicts worlds in a certain alignment that only occurs when the stormgate to El Dorado becomes accessible. If we follow this theory and take into account the diagram above showing what looks like a planetary alignment, then we should expect 6 planets to have a specific alignment that allows the stormgate to El Dorado to open. So, it looks like 1 large planet (possibly Valencia), 2 medium planets, and 3 small planets will align in this position. Just looking at the concept map, however, I can't find an alignment that exactly fits this diagram.
A remaining question that I have about the celestial bodies in the message is "the moon"; if Valencia is the "blue sun," then what world, if we're still understanding it as a metaphor, can be considered "the moon"? Could "the moon" be a reference to a world that takes place at night, such as Marleybone or Darkmoor? Or is "the moon" the second largest world, in which case it would be a toss up between Skull Island and Grizzleheim? Regardless of what "the moon" in the Celestian Message is, Marco Pollo emphasizes that it is "key" when the stars align.
As I mentioned above, cardinal directions are canon in the Pirate101/Wizard101 universe and seem to be applicable to the spiral as a whole. In fact, in "The Path" map (shown below), there is a compass included in the top left corner that does not appear in the concept map.
If the cardinal directions do remain the same across these maps, however, we have to sail east of Valencia, towards Aquila and Krokotopia, in order to find the stormgate to El Dorado, which is also depicted as being east of Valencia.
I think that this is very compelling because east of Valencia are 3 worlds that we (and Marco Pollo) have already marked as important:
Krokotopia, if you recall, is circled in red by Pollo and is the only labeled world not found on an orbital path. Krokotopia is also located on the same alignment as Valencia, meaning that its position in relation to Valencia is already important in discovering the hidden stormgate.
World #22, as previously mentioned, is positioned at the crossroads of two different orbital paths and is also circled in red by Marco Pollo.
Marleybone, addressed in our discussion of the map's coordinates, clearly has a position on the concept map that is deemed important by Marco Pollo, not to mention that it is the first world to appear in the list on the left side of the map. Furthermore, Marleybone has a moon in its sky- could Marleybone be "the key" referenced in the Celestian Message?