Add support for WPML post locale

[thomas-pike]

Adds WPML compatibility

Proposed changes:

• Gets the post locale from WPML Multilingual CMS plugin if installed

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Install the WPML Multilingual CMS plugin
• Publish a post or a translation of an existing post in a language other than the default site language.
• Check that the activity object is sent out with the expected language.

[Menrath]

This is a good first step. Wouldn't it be better to actually have one ActivityPub object, that contains all the languages in the contentMap? The current development in #1148 might be able to make this possible without much overhead.

[mattwiebe]

Thanks for the PR @thomas-pike !

[pfefferle]

I think this is a nice start and we can improve on this.