Sylvan is the language of the Sidhe.  To them, it's also known as Iredif-ylleth, which translates to True Speak.  It's believed that it was the Honest Folk that invented language, but it was the mortals who invented lies. 

You can't directly lie in Sylvan.  You can cleverly make your words seem to imply something you don't mean, you can lie by omission, and you can deceive in all kinds of ways, but you can't knowingly tell an untruth.  The words simply won't be able to leave your mouth.  If you know that what you want to say is untrue, you'll simply forget how to say it. 

If you make a promise in Sylvan, you're bound to your word.  

The full page has even more information. 

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Generator

byr, beth, bin, byf, byn, bei, ba, bas, baf, ber, bet, bes, bek, bel, ben, bit, bip, bid, bin, bim, bon, bom, bos, dyr, deth, din, dyf, dyn, dyx, dei, da, das, daf, der, det, des, dek, del, den, dit, dis, din, dim, don, dom, dos, fyr, feth, fin, fyn, fyx, fei, fa, fas, fer, fet, fes, fel, fen, fit, fip, fis, fin, fim, fon, fom, fos, heth, hin, hyn, hyx, hei, has, haf, hek, hen, hid, hin, him, hon, kyr, keth, kin, kyf, kyn, kyx, kei, ka, kas, kaf, ker, ket, kes, kel, ken, kit, kip, kis, kin, kim, kon, kos, lyr, leth, lin, lyf, lyn, lyx, lei, la, las, laf, ler, let, les, lek, lel, len, lit, lis, lid, lin, lim, lon, lom, myr, meth, min, myf, myn, myx, mei, ma, mas, maf, mer, met, mes, mek, mel, mip, mis, mid, min, mon, mos, nyr, neth, nin, nyf, nyx, nei, na, nas, naf, ner, net, nes, nek, nel, nen, nit, nis, nid, nin, nim, non, nom, pyr, peth, pin, pyf, pyn, pyx, pei, pa, pas, paf, per, pes, pek, pen, pip, pis, pid, pin, pim, pon, pom, reth, rin, ryf, ryn, ryx, rei, ra, ras, raf, ret, res, rek, rel, ren, rit, ris, rid, rin, rim, ron, rom, syr, seth, sin, syf, syn, syx, sei, sa, sas, saf, ser, set, sek, sel, sen, sip, sis, sid, sin, sim, son, tyr, teth, tin, tyf, tyn, tyx, tei, ta, tas, taf, ter, tes, tek, tel, ten, tis, tin, tim, ton, tos, vyr, veth, vin, vyn, vyx, vei, va, vas, vaf, ver, vet, ves, vek, vel, ven, vit, vip, vis, vid, vin, vim, von, vos, wyr, weth, win, wyn, wyx, wei, was, wer, wes, wel, wit, wid, win, wim, zyr, zeth, zin, zyn, zyx, zei, es, ed, ek, el, en, e, eda eka ena eni, is, id, ik, il, in, i isa ida ika ila ina as, al, an, a, asa, ala, ana, ani, ak, ol, o, on, ona oni, oth, qua, qui, y yar, yin, yath, yen, yv, yr, eth, in, yf, yn, yx, ei, a, as, af, er, es, el, en, is, in, im, on, os,

Copy and paste that, and then go here: https://scriboly.com/Generators/Language 

Syvlan Words

If you make a promise in Sylvan, you're bound to your word.  Here are some Sylvan phrases regarding deals and promises:

  • Es ademesif - I promise
  • Imseif es ad uset - There is the deal (both parties can say this as they shake hands)
  • Es rheeth then aa esif - I swear on my life (Be careful! You'll die if you break this!)
  • Es if ad iredim - I tell the truth (use to assure someone you're not lying)
  • Thir usif aa em - You have my word

Common Phrases

  • Theniflyem - The Honestfolk, used to refer to the race as a whole, or a group of them.
  • Thir Thenifly - Literally, "your honesty," use in place of "sir" or "madam" if they haven't told you what to call them.
  • Thir Esem - Literally "your wisdom," use this for a faerie of higher station, or to show deference. 
  • Thir Iffal - Literally "your beauty," use this if you're trying to flatter them; some might take offense to too much flattery, and some Unseelie might consider this an insult
  • Ifreth - sexy; use this while flirting
  • Deif if aeth se as - a greeting, "blessed be and merry meet"
  • Ek esif - literally "good times," used as an informal greeting
  • Emkyus - goodbye, literal translation is words meaning "go gracefully" mashed together
  • Kyusesedif - literally "gratitude," use as "thank you."
  • Imaeth - a less formal "thank you."  
  • Esif aa usthel es em thir - literally "like my back is to yours," it means "I'm with you" and "You have my support." 
  • Thir usesaethif emven! - literally "your radiance today!" but it's used as "you look nice today." 
  • Adreth - to help or assist

Swearing - all of these are curse words! Use carefully! 

  • Esimse ethven - Literally "wither away," this phrase is a curse
  • Eth wyles - eat shit! 
  • Vyusas / Esthel - vulgar words for the female and male genitalia, respectively
  • Ethek - a swear you use when you goof up, like "oh fuck" 
  • Edthel, edthelreth, edthelse, edthelif - fuck, fucking, fucker, fucked. Literally a vulgar word for sex. 
  • Es thir edthel ifseimreth? - Literally "did you fuck everything?" Use when you are displeased with someone, similar to "You fucked up"
  • Emwylesif - a very insulting word used for someone attractive but idiotic
  • Ases - a very insulting word for an idiot; it's such a strong word that it's a swear
  • Pyemur - something like "whore" or "slut," can be used for any gender
  • Adesthel - something like "prick" or "asshole," a not nice person. 
  • Iled - a useless, lazy individual 
  • Ethwylesif - a person who is no fun, a killjoy, not worth being here.  It also means "horrible dancer" but it isn't a swear when used that way.