translate: mangle is a bit useless due to rate limit

[RhinosF1]

Description

Mangle ends up just saying 'None' a lot after using it more than once or twice within a minute or so due to what seems to be a rate limit

Reproduction steps

1. Start a basic, stock sopel up
2. Use mangle a few times within a minute or so
3. It returns None

Expected behavior

It either back off and respond when it is successful or error nicely. @Rate (I think) might be useful here

Logs

If applicable, add logs to help us figure out what's happening. Raw logs are
super helpful! Logs are usually found in ~/.sopel/logs, depending on your
configuration.

Environment

• Sopel .version: lastest on PyPi
• Sopel installed via: pip
• Python version: 3.7
• Operating system: Debian Buster
• IRCd /version: freenode's
• Relevant plugins: mangle

Notes

https://cloud.google.com/translate/quotas

[dgw]

I had to spam a .mangle command over 20 times in a minute for it to stop responding.

Low priority issue, patches welcome, yadda-yadda.