It is up to individual students. Most study a module at a time, however in the case of language modules these are usually studied at the same time as one or more other modules because of the ongoing time involved studying a language.
The main regulations to bear in mind are whether some modules should be completed in a particular order. For example, at Level 6 students must end the course with the dissertation. They cannot begin Level 6 with this module. But in the case of electives these can be completed in any order a student wishes (unless there are pre-requirements).
These regulations are set out in the specification (syllabus) for each programme.