[DiogenesKynikos]

> Whether Taiwanese Mandarin is a separate language or not is a matter of opinion.

There are dialect pairs where the question of whether they are separate languages is blurry. Taiwanese Mandarin and the official Mandarin of the PRC are nowhere near the level of difference where this question even arises. Mutual intelligibility is unproblematic.

> the output for "zh-tw" does not reflect the language actually used by people in Taiwan, and to that end it betrays the trust of the user.

If users were actually selecting "Taiwanese Mandarin," as opposed to "Chinese (Traditional)," and if Google were doing a decent job of translating into Chinese in the first place, I would agree with you. But neither is the case.