Please give me a single word for: the ability of writing and speaking of a language.
Some of these choices have already been suggested in the other answers. I going to suggest few additional words:
i)basic ii) intermediate iii) professional working proficiency or business level iv) fluent. These words may be used to describe proficiency in each of the linguistic skill (writing, reading, speaking, listening).
You may also check out following resources: i) https://academic.oup.com/applij/article-abstract/34/1/99/150948?redirectedFrom=PDF