Allow fake source language translation

Problem/Motivation

The below is going to sound silly, but the situation we are in would greatly benefit from this.

Unless there is a way to change the base translation for an already translated node (EN original --> FR original), we have had to make a new node that is FR to start with. However, all the values being entered are going to be in English, and will need translation to French.

The ask here is to "fake" it or have TMGMT assume the existing text is in the language we specific in the "Enforce source language" setting during cart checkout.

Or any other way we can submit a TMGMT job with "EN" as the source and "FR" (the current node's base language) as the destination.

Steps to reproduce

- Select a node created in the FR language
- Translate tab -> add to cart
- Select "enforce source language" and choose English
- Set request language to French
- Receive error: From the selection you made it was not possible to create any translation job.

- Same, but try French to French.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes