Google Translate virtual keyboard can be useful, but I haven’t found any real documentation. Since it is needed when one writes in the translation panel, grabbing the mouse to open and use it, is not the best way. Here is what I found by experimentation. Assuming that one writes inside the source (left) panel of Google translate and the icon of the virtual keyboard is there, because the source language defined includes accents and diacritical marks, such as Greek, German, French, etc., focus can be given to the virtual keyboard icon by pressing two times the tab button of the physical keyboard, and then Enter.

Pressing the Enter button opens the virtual keyboard, but you can’t navigate from button to button using the arrow keys. Special characters are there, but you have to press them with the mouse, since you don’t know to what physical keyboard buttons they correspond. Here I give three buttons you can use on your physical keyboard, in order to have the virtual keyboard give very common German characters with umlaut.

Pressing a semicolon ( ; ) produces an o with umlaut (ö)
Pressing the single quote (the key left to the Enter button) produces a with umlaut (ä)
Pressing the left square bracket ( [ ) produces u with umlaut (ü)
The same keys with Shift produce these letters as capitals.

Note that placement of letters in the virtual keyboard corresponds not to somehow similar letters in the actual keyboard but to the physical placement of the keys themselves. For example, if the virtual keyboard shows letter x to be on the first row, third key, then to write it you need to press the number “2” of the physical keyboard.