With the selection or finger tool - red framed - you can select and highlight a country.
Here, in year 1000, West Francia is selected and thus appears highlighted in grey and surrounded by a thick black line. Note that the name 'West Francia (=short form), Kingdom of the West Franks (=long form)' is selected in the entities list, on the left, and appears again just below the current century. Only big cities are shown on the main map because of the low zoom level.
Euratlas Periodis Expert is available either in English or in French or in German.