@Lordy Hello, you can change this in the MMM-COVID19.js (you find it in /MagicMirror/modules/MMM-COVID19) .

   .......
    headerconfirmedCell.innerHTML = 'Confirmed'
    .....
    headerNewConfirmedCell.innerHTML = 'New Cases'
    .....
    headerdeathsCell.innerHTML = 'Deaths'
    ......
    headerNewDeathsCell.innerHTML = 'New Deaths'
    ......
    header.recoveredCell.innerHTML = 'Recovered'
    ......
    headeractiveCell.innerHTML = 'Active'
....

Just type in the “” the text you would like to see on the module.

i have translated it into german

headerCountryNameCell.innerHTML = ''
    headerconfirmedCell.className = 'number confirmed ' + this.config.headerRowClass
    headerconfirmedCell.innerHTML = 'Bestätigt'
    headerNewConfirmedCell.className = 'number confirmed ' + this.config.headerRowClass
    headerNewConfirmedCell.innerHTML = 'Neue Fälle'
    headerdeathsCell.className = 'number deaths ' + this.config.headerRowClass
    headerdeathsCell.innerHTML = 'Tote'
    headerNewDeathsCell.className = 'number deaths ' + this.config.headerRowClass
    headerNewDeathsCell.innerHTML = 'Neue Tote'
    headerrecoveredCell.className = 'number recovered ' + this.config.headerRowClass
    headerrecoveredCell.innerHTML = 'Erholt'
    headeractiveCell.className = 'number active ' + this.config.headerRowClass
    headeractiveCell.innerHTML = 'Aktiv'

Just replace the part.
Ersetze einfach die Stelle