The Colloquial series has many different languages, including some that are normally very difficult to find resources for. Each Language has different authors. It consists of a coursebook and audio. The courses attempt to give the learner a better sense of the everyday speech style of a particular language.
Despite the title, the courses usually introduce the learner with standard but modern language.
By their method, they belong among the more conservative courses, like Teach Yourself series they are often being compared to, which is not necessarily a bad thing. Quite the opposite, most learners choose it because of it structured style.