Allow importing translations from the new PHP format

[akirk]

What problem does this address?

In #1626, only the export of translations was implemented. Importing originals and translations was disabled.

What is your proposed solution?

We can import translations but not originals. The latter has been deemed not feasible in #1626 since the PHP format doesn't have comments or line references, but for translations it could be done.

This issue is just a note that this could be implemented, so far nobody from the community has shown the need to import the PHP format. We can look into this when we see people need this.