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.
关于外国文字的问题：字母或文字须符合各语言用法。例如，“ж”是俄语字母，但中文不会用到，因此不应出现在原始文本中。