Language Centres within universities can often use the university quality assurance procedures, documenting their programmes with module and course descriptors, following standard practices in marking and appointing external examiners to oversee their assessment processes.
Another way is for the institution to use an external exam such as IELTS or TOEFL as the assessment so that quality is then the responsibility of the external body which sets and marks the exam.
A third way in the UK is for language centres within universities to seek accreditation from bodies such as the British Association for Lecturers of English for Academic Purposes. BALEAP runs an accreditation scheme which evaluates the quality of the course design, teaching and learning, assessment, management, resources, student welfare and support and ongoing evaluation of the programme.
The institution prepares documentation in advance which describes the programme under these different headings and then two assessors arrange to visit, observe classes and talk to the management about their programme. In my experience this is a very good way to get everyone associated with the programme, from senior management down to the temporary teachers to 'lift their game' and commit to higher quality.
Have you had experience of something similar in your context?