This is a browser extension provides some keyboard shortcuts in google translate page, but these shortcuts are not customizable.
For Firefox: https://addons.mozilla.org/zh-TW/firefox/addon/google-translate-shortcuts/
- Alt + O = focus the input text
- Alt + C = copy the output text
- Alt + J = left audio button
- Alt + K = right audio button
- Alt + L = clear the input text
- Alt + M = switch to original language
- Alt + S = swap language
- Alt + Shift + A = detect language
- Alt + Shift + Q = switch to left lang1
- Alt + Shift + W = switch to left lang2
- Alt + Shift + E = switch to left lang3
- Alt + Shift + U = switch to right lang1
- Alt + Shift + I = switch to right lang2
- Alt + Shift + O = switch to right lang3
- ` (Grave accent) = focus the input text
(If it doesn't work on Firefox, you can give the "Access your data for all websites" permission to make it work.)