Words for sweet and related things in Celtic languages.

Words marked with a * are reconstructions.

Proto-Celtic	*melissis, *melisti = sweet
Old Irish (Goídelc)	milis [ˈmʲilʲisʲ] = sweet
Middle Irish (Gaoidhealg)	milis = sweet, fresh (water)
Irish (Gaeilge)	milis [ˈmʲɪlʲɪʃ] = sweet, tender, tasty, fresh (water); honeyed (words), flattering milisbhriathrach = sweet-spoken, honey-tongued milisín = sweet morsel milseán = sweet, bonbon, candy; sweet dish, dessert milseog = dessert; sweetheart, darling milsigh = to sweeten uisce milis = sweet/fresh water teange mhilis = flattering tongue
Scottish Gaelic (Gàidhlig)	milis [milɪʃ] = sweet; melodious, musical; flattering aran-milis = gingerbread baine milis = sweet milk, condensed milk buntàta milis = sweet potato, yam coirce-milis = sweet corn, maize slaman-milis = jelly
Manx (Gaelg)	millish = sweet, darling, sugary, dulcet, honeyed, luscious, fruity, balmy bee millish = sweetmeat, sweet feddan millish = recorder, fipple flute fooillagh millish = trifle slumm millish = jelly ushtey millish = fresh water focklyn millish = honeyed words
Proto-Brythonic	*melɨs = sweet
Middle Welsh (Kymraec)	melis, melys = sweet, pleasant-tasting, delicious meledic = sweetened with honey melysvwyt, melysvwyd, melys-fwyd = sweet food, sweet dish, delicacy, dainty food, sweetmeat, dessert melûsrwyth, melyssrwydd = sweetness, sweet or delicious taste
Welsh (Cymraeg)	melys [ˈmɛlɨ̞s/ˈmeːlɪs/ˈmɛlɪs] = sweet, pleasant-tasting, delicious, tasty; not salty, fresh (water); delightful, agreeable, pleasant, charming; sweet-sounding, euphonious, melodious; sweet-smelling, fragrant dant melys = sweet tooth pethau melys = sweets tatws melys = sweet potato melysaf, meluso = to sweeten, mitigate, alleviate melysaidd, melusaidd = rather sweet, delightful, agreeable melyfwyd, melusfwyd = sweet food, sweet dish, delicacy, dainty food, sweetmeat, dessert melyfwyd, melusfwyd = sweet, delicious, delightful, agreeable, pleasant, melodious, fond of sweet food or drink, greedy, voracious melysrwydd, melusrwydd = sweetness, sweet or delicious taste, melodiousness, fondness for delicate food
Cornish (Kernewek)	melys [‘mɛlɪs/’mɛləʃ] = very sweet, honeyed kleves melys = diabetes
Old Breton (Brethonoc)	milis = honeyed
Middle Breton (Brezonec)	milis = honeyed, sweetish, sneaky
Breton (Brezhoneg)	melis, milis = honeyed

Etymology: from Proto-Indo-European *mélid (honey)), which is also the root of the English words mildew and mulch [source].

Proto-Celtic	*su̯eko-s = to smell (good)
Middle Welsh (Kymraec)	chwech = sweet wech, chweg, chwec = sweet, lucious, pleasant, comely
Welsh (Cymraeg)	chwech [χweːχ/hweːχ] = sweet chweg = sweet, lucious, pleasant, comely chwegaidd = sweetish chwegrwydd = sweetness, lusciousness
Middle Cornish (Cernewec)	hwec, hwecah, hweg, hwêg, whêk = sweet, pleasant, dear hwecter, whecter = sweetness, pleasantness, suavity, delight hwegol, whegol = dear
Cornish (Kernewek)	hweg = sweet, dear, gentle, kind, nice, pleasant, pleasing hwegednow, hwegnnow = candy hwegen = darling, pet, lozenge, sweet hwegoll = darling, delightful, sweetest hwegys = sweetcorn, maize hweg-oll = darling, delightful, sweetest ys hweg = sweetcorn, maize
Middle Breton (Brezonec)	huec = sweet, delicate
Breton (Brezhoneg)	c’hwek [xweːk] = soft, suave, sugar, delicious, exquisite, pleasant c’hwekaat = to sweeten c’hwekted [ˈxwek.tet] = softness, suavity c’hwekter = delicacy mel c’hwek = sweet honey gwin c’hwek = sweet wine

Etymology: unknown

The usual Breton word for sweet is dous, which probably comes from the Old French dous (soft, tender), form the Latin dulcis (sweet, fragrant, melodious), from the Proto-Indo-European *dl̥kú- (sweet) [source].

Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie, eDIL – Electronic Dictionary of the Irish Language, In Dúil Bélrai English – Old Irish glossary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionaire Favereau, TermOfis, English – ProtoCeltic WordList (PDF), Etymological Dictionary Of Proto Celtic