A card is composed of a template and its note’s fields which are referenced by the template. So every piece of information on the card must be either stored in the template or a note field.
You could have a field like this:
1. <span class="hover">Portugal</span>
<span class="hint"> [population around 10 million]</span>
, 2. <span class="hover">Poland</span>
<span class="hint"> [capital is Warsaw]</span>
, 3. <span class="hover">Peru</span>
<span class="hint"> [country in South America]</span>
And implement the hover functionality in the template styling.
Not sure if this is what you’re looking for as it would show the same hints for all templates the field is used in. If you need template-specific hints, you have to add a similar field like the above for every template.