As a rule, client-side QA testers perform functional testing. It partially overlaps with the linguistic testing, but it only checks the obvious things such as formatting and encodings. Other specialists (LQA- or LQT-testers) are responsible for full localization testing.
What a localization tester must be able to do
LQT-tester should be able to read test-cases, to log errors in the form requested by the client, to record video and use emulators to test the localization on the device model specified by the client.
Seemingly, the task is easy and can be done by any native speaker. This work is usually done by the translators who worked on the project, because testing allows them to fully immerse themselves in the context of the game. If there are no questions about the context of a line during the translation, it may turn out that this context was important but was hidden in the game. At the LQT stage it is possible to edit any line of text to make it fit more organically into the game situation or the interface.
But it also happens that on one project a translator is engaged only in localization, on another – only proofreading localized strings, and on the third – localization testing.
There is also a vendor manager and a specialist in the localization of audio and video content, but these are even more niche professions. Perhaps, the statement about the value of narrowly-specialized translators can be extrapolated to localization specialists in general. The industry is amazingly malleable, and in-demand professionals are born at the junction of competences.