To reproduce:

1. Translate an article from English to Portuguese.
2. Click a paragraph to translate it. Wait for the machine translation to load.
3. In the "Tradução inicial" box ("Initial translation"), click "Usar Google Translate" ("Use Google Translate").
4. Select "Usar Yandex.Translate". Wait for the machine translation to reload.
5. Click "Manter como padrão" ("Keep as default").
   1. Observed: The whole Initial translation box disappears. I'd expect it not to disappear. (@Pginer-WMF, please check what does the design specification say.)
6. Click another paragraph. Wait for the machine translation to load.
   1. Observed: The "Tradução inicial" box ("Initial translation") shows "Usar Google Translate". I'd expect it to say "Usar Yandex.Translate".

I tested it with Ukrainian, Russian, and Portuguese.