Working memory is “a brain system that provides temporary storage and manipulation of the information necessary for . . . complex cognitive tasks” (Baddeley, 1992, p. 556).
Your question depends on what working memory capacity test you are looking for and what you want to measure. The size comparison span (SICSPAN) test e.g. is used to assess L1-WMC and L2-WMC because SICSPAN is known to be a good predictor of speech processing abilities (e.g., Sörqvist & Rönnberg, 2012). Hpoe you find what you are looking for.
Nback tasks are often used to test and train working memory. They are utilized as a gold standard in cognitive research, and are part of many standard cognitive training programs such as the APT (Attention Process Training) which is one of the most research cognitive training programs. They are also readily set up in most stimuli presentation software packages. If you are using the N-back to train working memory, you should use a different measure to test the functional generalization, otherwise you are just testing their learning of one task.