Right radical 月 — the moon itself, here the actual celestial moon (NOT the look-alike ⺼ meat radical). The moon is the source of brightness in 朗: the character paints a clear moonlit sky, hence 朗 means clear, bright, by extension cheerful (开朗, 朗读).
Left side 良 — supplies the sound (liáng drifted to lǎng) and a complementary meaning. 良 means 'good, fine,' and a clear bright moon is a good moon. Drawn here in a 6-stroke contraction of standalone 7-stroke 良 (the bottom dot absorbed). A good clear moon = bright.
