Without seeing the the project or bundle it will be difficult to analyze the root cause. In projects that are just bi-lingual you cannot add another reference language to UI. In bundles it is not possible at all. Reference languages can be added by project managers that are exporting the bundles.
In all my multilingual projects I'm able to add a reference language. Please note that this function is language specific, so the translation list must be open to select the function. When the project view is displayed the ribbon entry can be greyed out.
If it doesn't work on your end I would suggest to log a case so that the support engineers can help by directly looking onto your system.
I had tried that before, but now I see that the "Reference Language" selection in the ribbon - which was greyed out, didn't disappear as it seemed. It changed into an icon, which looks like it works okay.
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