Though I’ve already created the issue, I publish this here to warn those using the Crossref module to export metadata in multilingual journals:
The title says it all (see also the issue description). The only workaround by now is to choose the locale according to the submission language of the metadata being exported. Anyway, the problem is submission language may differ for some articles.
So my opinion is that language of metadata exported should depend NEITHER on submission language NOR on locale. That is, it may depend on submission language by default. Though the journal manager should be able to override this and to specify the language of metadata exported explicitly.
Generally, Crossref encourages journals to export all metadata in English, so this option would be really important.