[MC-265792] Names of letter and number keys appear to be untranslatable

The bug

The names of all 26 standard letter keys (A-Z) and the ten number row keys (0-9) do not appear to be translatable, even though all other standard US keyboard keys are.

How to reproduce

Run this command, replacing ? with any of the 26 letters of the alphabet (lowercase) or 10 digits:
```
/tellraw @s {"translate":"key.keyboard.?"}
```

Expected results

In chat, a single letter or number would be displayed corresponding to what you put in.

Actual results

The entire string key.keyboard.? is displayed instead, implying it does not get translated anywhere.