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.
Le litteras debe esser valide in le lingua a parlar. Per exemplo, “ж” es un littera in le alphabeto russe ma non es jammais usate in anglese e dunque non deberea jammais apparer in alcun texto fonte anglese.