HFP-3943 Use user's profile language instead of site language

[otacke]

When merged in, will use the users' profile language for the H5P editor instead of using the site's language which can differ.

Note: The use of WPLANG is deprcated.

[icc]

What's the benefit here compared to using get_locale()?

[otacke]

@icc get_locale will give you the language that the site is set to, but it is not necessarily the one that the user has set for their backend. WordPress could e.g. use English for an international audience, but users can override this to be their native language for them. This is not something that I came up with, but it was a request from someone running into that specific problem.