Could you tell me what is known about those two continents?
I don't know if there is much information abouts these two
but I would like to find out more about them.

PS:I did read the UESP-Articel.

Pyandonea is a land to the south of the Summerset Isles, and the land where the Paquiniel as related to Topal the Pilot likely sailed. The locals are the Maormer, white-skinned white-eyed Tropical Elves and Sea Elves who sail in ships made of insects and who ride Sea Serpents. Their leader, King Orgnum, is an immortal wizard who is constantly growing younger (and I have a personal theory that 'King Orgnum' is in fact a series of 'King Orgnum's). The Maormer have some degree of control over the mist storms that swamp their land, using it to catch enemies unawares and using it as a simple barrier to entirely block potential enemies from entrance into their kingdom; they have used their mists as such a barrier for five hundred and twenty-four years, entirely isolating Pyandonea from the rest of the world, or at least from Tamriel. It is possible that the Maormer were one of the very first elven races to split off from the Aldmer, as they were most certainly not ejected from the Altmer. King Orgnum is, at times, compared to the Serpent God Satakal. They have six-herbs, and King Orgnum gave some to Potema before engaging in, ah, recreational activities with her. Some people seem to think that they ride turtles, particularly some Kragius guy on this forum, but I've never seen lore evidence for that (I'm working on a project related to them, and just yesterday I scrolled through every mention of the Maormer ever on this forum. The Kraigus or Kragius guy brought up the turtle in half the posts about them, but I still haven't been able to find the source of this thought of his).

Pyandonea is a sea-jungle, a collection of islands that are often so tightly packed together as to make some branches of the sea seem to be canyon rivers. These islands don't have welcoming beaches, no. Instead, they have hostile cliff faces, and each island is crowned by jungled green. There is reaching kelp in the water that stalls all ships but those of the Maormer. The Maormer are experts of something called 'Snake-magic'. It is a personally theory of mine that the Sea Serpents of Pyandonea are inverse-Dragons, ie they are the Space God's answer to the Time God's dragons.

Anything else?

Ok thanks.
Can someone tell me something about Yokuda?

Yokuda is the home land of the Redguards, who were ancestral enemies with the Left-Handed Elves, whom we don't know much about, other than that they built the Orichalc Tower.

For reasons unknown precisely, Yokuda is gone, destroyed, forcing the Yokudans to leave their homeland. Some say it was an earthquake (boring therefore wrong), others that an ancient technique of the Ra'gada sword masters caused the devastation (awesome).

I'm afraid we don't know much else.

Thanks,
sad that there is not known more about Yokuda.

Apparently there's still an archipelago where it once stood, with some shipping between it and Tamriel, so there must still be settlements there.

Now, what I want to know is where is Thras located in relation to Yokuda, Pyanodea, and Alinor?

http://media.giantbomb.com/uploads/0/3508/716027-west_centraltamriel3.jpg Thras is not mentioned on the map, and I can't seem to find a larger version of this specific map, but http://media.giantbomb.com/uploads/11/116554/1975232-tamrieltopo.jpg (which looks like a modder resource) claims that it is composed of Vandor, the Sestres, and presumably other islands too small for such a large map. The conclusion that those islands are the islands of Thras is well founded, considering that those islands are roughly southwest of the Chain, and PGE 1 claims that Thras is just that, "The coral kingdoms of Thras, a set of islands southwest of the Chain in the Abecean Sea."

Aye, the remnants of Yokuda are known as the Orichalc Islands, likely after Orichalc Tower. It features prominently in http://imperial-library.info/content/lord-vivecs-sword-meeting-cyrus-restless, which I recommend to anyone who likes a good story.