From a technical standpoint this is relatively easily to implement. We use the existing OOUI ComboBoxInputWidget. Items can have labels that are different from the submitted value. The widget even supports icons and item groups. The complex part of this task is to decide if and how exactly we want to support this. Problem is: once decided it's very hard to change the TemplateData specification.

As of now, suggested values look like this in the TemplateData JSON:

"suggestedvalues": [
    "Germany",
    "Poland"
]

This is how it might look like with labels:

"suggestedvalues": [
    {
        "data": "de",
        "label": "Germany"
    },
    {
        "data": "pl",
        "label": "Poland"
    }
]

• The old format should still be allowed. The two can even be mixed.
• The "data" and "label" keys follow OOUI terminology.
• "data" must be required.
• "data" should be a string. Numbers don't make much sense here. However, the string should be allowed to be empty.
• While we could check for uniqueness, there is probably not much value in doing so. The harsh way TemplateData reports validation errors should only be used when the JSON is not safely usable otherwise. This is not the case here.
• We might want to make "label" optional. It defaults to what "data" says then. Again, this follows existing OOUI specifications, specifically the behavior of TagMultiselectWidget.
• "label" should follow the existing definition of InterfaceText.
• The old format with an array of strings can not support InterfaceText, as this would clash with the new format.
• Later iterations can e.g. add an "icon".