I propose to change the justify-content: left; to justify-content: flex-start; for the Modal block heading alignment.

With the proposed change there's no need for extra RTL styling adjustment to handle Modal's heading alignment.

.components-modal__header-heading-container {
  align-items: center;
  flex-grow: 1;
  display: flex;
  flex-direction: row;
  justify-content: left; // here
}