Usability
Interface 
User-friendliness/complexity of the tool´s interface
Straightforward, intuitive.
Time efficiency Time needed for mastering the „how-to“ of the tool and a production of a task
Medium; high if using ready-made flashcards.
Interface language(s) The number of languages in which the tool is currently available/adaptable to
Several languages, including English, German, Spanish, French, Italian, Portuguese, Russian, Mandarin, Korean, Polish, and more.
Respect for privacy Does the tool require registration? Who can see my data? Can I remain anonymous?
Medium. Quizlet collects different personal information, including account information, transactional information, device information, and more. It also gathers cookies for different purposes, among which marketing. Quizlet employs industry-standard security measures to protect user data. It acknowledges users’ rights to review, edit, or delete their data, as well as to delete their accounts. Quizlet states compliance with the GDPR. See more here and here.
Saving work for future use Possiblity to save your or others´ work for reuse/reproduction
Yes, with an account.
Tailorability (If so, how?) Possibility of the tool to be used for a range of different activities (e.g. to what extent you can organize a project assignment with a central activity based on this tool)
Yes; Quizlet provides quizzes for a great variety of subjects and can be used by learners at different levels.