How do you say "immersion" as in "full immersion language program"?
Although in a 洗禮(baptism), a person may be immersed in water for a brief moment, but the purpose of baptism is mostly for symbolizing purification or regeneration. For example: 經過戰火的洗禮 (After the baptism of war). It is not a very good translation for "immersion" in this case.

沉浸 is the correct translation for "immersion"

"Full immersion language program" in Chinese should be 全浸式語言課程

Another suggestion :

單語環境學習課程 (single language environment study course)