I had no luck trying to set the correct language. Do you have an example how it's solved in oxygen?
I'm sorry but I do not quite understand your setup. Please give me more details. Are you working with a plugin, with a framework or both? The API I gave you was to find out the application language used by Oxygen. Does it not work for you or are you having another use case?
If you work on the framework level you can use the internationalization support we have at framework level:http://www.oxygenxml.com/doc/versions/17.0/ug-editor/#topics/localizing_frameworks.html
Unfortunately we do not have this support when creating a plugin. But you do have the API to access the current user interface language and you can create your own resources bundle.
Another question about localization: I can translate the actions but not the css and xslt titles. Is it possible to translate those entries to get an fully localized framework?
By css do you mean the titles for the alternate CSSs? Indeed, they cannot be localized but I could try and add an issue to fix this.
By XSLT titles do you mean the transformation scenario names? They are also not localized, maybe we could also consider localizing them.