[Cashtab] prevent translation of user mnemonics

Summary: Indonesian user reported translation of mnemonic resulting in backup difficulty. The long term solution is to support multiple languages and mnemonics in multiple languages. However, until this is complete, English mnemonics should be preserved in English.

Test Plan:

npm start

Navigate to localhost:3000 in Chrome
Go to Settings page and view your mnemonic
Right click on page "translate" and select any other language
Confirm that the mnemonic is not translated

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9785