Krita is an open source desktop painting application designed for illustrators, concept artists, illustrators, comic book creators, matte painters and texture artists, and the VFX industry. Krita is used daily by professional artists all over the world, as well as enthusiastic amateurs. Krita comes with translations in many language.
There are great brush engines for sketching and painting, stabilizers for freehand inking, assistants for constructing a complex scene, a distraction-free canvas-only mode for painting without bother, clone layers, filter and transform masks for non-destructive editing. You can exchange files with other applications in many different formats.