Add the currency the Direct Stripe donation button is set with.

ds-donation-currency.php

<?php
//We add a span tag that holds the currency text corresponding to a donation button
add_filter( 'direct_stripe_donation_input', function( $direct_stripe_donation_input, $instance, $button_id ) {
  if( $button_id === 'ds-dons' ){ // $button_id is the CSS ID set in the button settings
    $text = 'EUR';
  } else if( $button_id === 'ds-donation' ){ // $button_id is the CSS ID set in the button settings
    $text= 'USD';
  }
  $direct_stripe_donation_input .= '<span class="curency-info">' . $text . '</span>';
  
  return $direct_stripe_donation_input;
}, 10, 3);