BitmapFont string render to an image file.

StringRender.java

BitmapFont font = interfaceService.getSkin().getFont("default-font");
GlyphLayout glyphLayout = new GlyphLayout(font, getString("langZhTw"));

final int w = MathUtils.ceil(glyphLayout.width + 2f);
final int h = MathUtils.ceil(glyphLayout.height + 2f);

OrthographicCamera cam = new OrthographicCamera(w, h);
cam.setToOrtho(false, w, h);
cam.update();

Batch batch = new SpriteBatch();
batch.setProjectionMatrix(cam.combined);

FrameBuffer frameBuffer = new FrameBuffer(Pixmap.Format.RGBA8888, w, h, false);
frameBuffer.begin();

Gdx.gl.glClearColor(1f, 1f, 1f, 0f);
Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

batch.begin();
font.draw(batch, glyphLayout, 1f, h-1f);
batch.end();

ByteBuffer buf;
Pixmap pixmap = new Pixmap(w, h, Pixmap.Format.RGBA8888);
buf = pixmap.getPixels();
Gdx.gl.glReadPixels(0, 0, w, h, GL20.GL_RGBA, GL20.GL_UNSIGNED_BYTE, buf);

frameBuffer.end();

PixmapIO.writePNG(Gdx.files.absolute("D:/zh-tw.png"), pixmap);

batch.dispose();