Amendments can be made to machine-translated text

[BigSnicker]

Issue: Amendments can be made to machine-translated text. This could distort the original proposal text. Amendments should only be possible in the submitted (i.e. original) language of User Proposals. Note: Amendments can be made to either side of Official Proposals.

To reproduce:

1. Take a proposal that has machine-translated text (https://staging.deploy.green.ca/processes/Policy2021/f/223/proposals/2583/?locale=fr)
2. Propose an amendment (e.g. "Modifier Proposition")
3. The proposal text opens with the machine translated text.

The above proposal body text is machine-translated, and so should not be amendable.

[BigSnicker]

This is an amendment that was written based on the machine-translated text of a proposal: https://staging.deploy.green.ca/processes/Policy2021/f/223/proposals/2587

[paarals]

closed via #153