Formal: 我很高兴 (Wǒ hěn gāoxìng) - This is the most common and formal way to express happiness.
Informal: 我开心 (Wǒ kāixīn) - This is a more casual way to say "I am happy" and is often used with friends and family.
Both translations convey the same meaning, but the choice depends on the level of formality and the relationship between the speaker and listener.