There are more sylvaneth than just the treespirits, sprites (spites in AOS) are sylvaneth, the spirits that possess deadwood to forge bodies out of it (treekin) are sylvaneth, the ghosts and fae creatures of the glades are sylvaneth. The disparity in sentience can be pretty huge between different castes. These days most castes are warrior castes.
Most Sylvaneth do not like mortals.
All Sylvaneth are unfathomably long lived.
Sylvaneth build cities, work metal and stone and create artworks, tapestries and furniture.
The song of Sylvaneth helps bind them together as a united people. It stops them struggling against one another not based on oppression but genuine feelings of unity.
Sylvaneth in general don't care for glory, expanding borders or making money. They're more or less solely concerned with ensuring the safety and cycles of the natural world are upheld.
They're difficult to interact with, not just becase of the language barrier. They do not like you, they don't think like you and they don't live and die like you. Your concerns are extremely alien to them just like they are to you.
They consider "Quickbloods" (mortals/demons/non sylvaneth in general) to be short sighted, flippant, brief and unable to comprehend the importance of their extremely long term plans and the real meaning of everything.
They ARE good people. With noble desires and despise chaos. it's just that they're so alien to us that the alliance is always quite strained.
There are tribes of humans that worship sigmar and alarielle living in ghyran that are tolerated by the Sylvaneth. There are also elves and dwarves.
Wanderers are HATED by the Sylvaneth. They're essentially living out a mission of repentance to Alarielle, planting waystones so that the forests of Ghyran can sprout in other mortal realms.
Not all sylvaneth sprouted in GHyran, they hail from almost every one of the mortal realms, even the realm of metal.