I live in a polylingual country, and I speak all four public languages, mostly rather badly. In consequence, I find translation apps essential. I will translate in both directions; translate a word from one language, then swap languages to verify a construction using that word in the other. For a translation app to be useful, it must support quick swapping of languages. This requires a swap button.
This is why I am more than a little irritated that my old go–to translation app, Bing on the iPhone, has lost that button. Suddenly, to exchange languages, I had to change the values of two drop downs (now fixed), then copy & paste the text across. This has made what was quite a useful app into a chocolate teapot. It’s now uninstalled.
Fortunately for me, the google translation app doesn’t have this design error. It has that swap languages button. In consequence, I now rely on it.