Letters must be valid in the language being spoken. For example, “ж” is a letter in the Russian alphabet but is never used in English and so should never appear in any English source text.
E lettie addeuviæ an da ëse valide inta lengua ch’a se parla. Pe exempio, «ж» a l’é unna lettia de l’alfabeto çirillico, a no l’é mai addeuviâ in ligure e donca a no s’à da addeuviâ inti testi liguri.