.: Concerning Green-elven Language and Writing :.

"The Silvan Elves (Tawarwaith) were in origin Teleri, and so remoter kin of the Sindar, though even longer separated from them than the Teleri of Valinor. They were descended from those of the Teleri who, on the Great Journey, were daunted by the Misty Mountains and lingered in the Vale of Anduin, and so never reached Beleriand or the Sea. They were thus closer akin to the Nandor (otherwise called the Green-elves) of Ossiriand, who eventually crossed the mountains and came at last into Beleriand."
   - Unfinished Tales, "The History of Galadriel and Celeborn, Appendix A: The Silvan Elves and their Speech"

"But the speech of the Green-elves in Ossiriand became somewhat estranged from that of their own kindred that remained east of Ered Lindon, being much affected by the tongue of Thingol's people. Yet they remained apart from the Telerian Ilkorins [Úmanyar] and remembered their kin beyond the mountains, with whom they had still some intercourse, and named themselves in common with these Danas. But they were called by others Green-elves, Laiquendi, because they loved the green wood, and green lands of fair waters; and the house of Denethor loved green above all colours, and the beech above all trees. They were allied with Thingol but not subject to him, until the return of Morgoth to the North, when after Denethor was slain many sought the protection of Thingol. But many dwelt still in Ossiriand, until the final ruin, and held to their own speech; and they were without a king, until Beren came unto them and they took him for lord. But their speech has now vanished from the earth, as have Beren and Luthien. Of their kindred that dwelt still east of the mountains few came into the history of Beleriand, and they remained in the Hither Lands after the ruin of the West in the great war, and have faded since or become merged among the Lembi [Avari]. Yet in the overthrow of Morgoth they were not without part, for they sent many of their warriors to answer the call of Fionwë [Eönwë]."
   - The Lost Road and Other Writings, "Valinor and Middle-Earth Before the Lord of the Rings, Quenta Silmarillion: Of the tongues of the Elves in Middle-earth..."

Language

Unfortunately, Professor Tolkien never developed Nandorin, the language of the Nandor (a.k.a. 'Danians', from their leader Lenwë/Dân, father of Denethor). These were Elves of the Teleri who broke from the Great Journey, therefore Nandorin would have been a branch of Common Telerin. The Green-elves, in turn, were a branch of the Nandor who settled in Ossiriand (now Lindon.) Apparently, they spoke 'West-Danian' (also called Ossiriandrin or Ossiriandish¹), while those who remained in the East (the Silvan Elves) spoke 'East-Danian'.²

However, on RealElvish.net Fiona Jallings, a professional Tolkien linguist, describes Woodelven Sindarin as follows:

This dialect of Sindarin was originally spoken in south-eastern Beleriand – Lindon and Ossiriand. The survivors moved to Mirkwood and Lothlórien, joining their Nandorin cousins. The Nandor adopted this dialect of Sindarin, making it the Woodelven dialect.

Perhaps the East- and West-Danian dialects were not very different in form, but more a case of differing accents. After all:

"In Lórien at this period Sindarin was spoken, though with an 'accent', since most of its folk were of Silvan origin. This 'accent' and his own limited acquaintance with Sindarin misled Frodo..."
   - LOTR, Appendix F, "The languages and peoples of the Third Age"

Therefore, I reckon the vernacular of Feveren's kindred would be a dialect of Ossiriandrin and can be safely rendered in Fiona's Woodelven Sindarin, which they would use together with the Common Sindarin tongue that they learned from the Grey-elves of Harlindon. As in Lothlórien, Westron would probably be uncommon among the Lindar; as Haldir explains in The Fellowship of the Rings:

'We seldom use any tongue but our own; for we dwell now in the heart of the forest, and do not willingly have dealings with any other folk. Even our own kindred in the North are sundered from us. But there are some of us still who go abroad for the gathering of news and the watching of our enemies, and they speak the languages of other lands.'

Writing

Although the dialects of the Silvan Elves, when they again met their long separated kindred, had so far diverged from Sindarin as to be hardly intelligible, little study was needed to reveal their kinship as Eldarin tongues. Though the comparison of the Silvan dialects with their own speech greatly interested the loremasters, especially those of Noldorin origin, little is now known of the Silvan Elvish. The Silvan Elves had invented no forms of writing, and those who learned this art from the Sindar wrote in Sindarin as well as they could. By the end of the Third Age the Silvan tongues had probably  ceased to be spoken in the two regions that had importance at the time of the War of the Ring: Lórien and the realm of Thranduil in northern Mirkwood. All that survived of them in the records was a few words and several names of persons and places.
   - Unfinished Tales of Numenor and Middle-earth, "Appendix A, The Silvan Elves and Their Speech"

Initially Tolkien wrote that the Runes (Cirth) were invented in Beleriand by the Nandor³ (i.e. the Green-elves) in the First Age, but in the published Appendix E ("Writing and Spelling") of The Lord of the Rings he instead states that they were devised by the Sindar of Doriath.⁴ Much lore was lost in the ruin of Beleriand, so I assume that the surviving Iathrim and Laegrim would each have their own tale regarding its origin. (Perhaps the ancient Cirth that Daeron standardised into the Certhas Daeron, and subsequently expanded into the Angerthas Daeron, were indeed Danian.)

At any rate, by the Second Age they had fallen from common use, and by the Third Age were rarely used by elves. For the purposes of Feveren's tale, I'll assume that they had been largely forgotten by the Green-elves of Ossiriand who remained in Lindon, but after almost six and a half thousand years of mingling with the Grey-elf refugees from Doriath, the circumstances would be similar to that of the eastern Silvan Elves.

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1. "Words, Phrases and Passages in Various Tongues in The Lord of the Rings", Parma Eldalamberon XVII

2. "Tengwesta Qenderinwa and Pre-Fëanorian Alphabets Part 2", Parma Eldalamberon XVIII

3. "...the Runes, or Cirth as they were called, were first devised by the Danians... in the woods of Beleriand, and were in the beginning used mainly for incising names and brief memorials upon wood, stone, or metal."
   - The Peoples of Middle-earth, "The Appendix on Languages'"

4. "The Cirth were devised first in Beleriand by the Sindar, and were long used only for inscribing names and brief memorials upon wood or stone."
   - The Lord of the Rings, Appendix E: 'Writing and Spelling.'