Q) How to say 'Bon Appétit' in Korean? + Other expressions to say when starting to eat?

Q) How to say 'Bon Appétit' in Korean?

Normally we say, 

1. 잘 먹겠습니다. (Formal form but!! It's a diary form and you can use it when you are with friends, colleagues, parents, inlaws, basically whoever you are with.)

Note: 잘 (well) + 먹겠습니다 (from 먹다, to eat) = I will eat it well.

You can also say these in different situations. First, when someone offers you a meal.

1. 잘 먹을게. (Informal; When your friend or person who is younger than you buys or cooks a meal for you, you can say this. If the person who buys or cooks a meal is older than you, you can simply say 잘 먹겠습니다.)  = I will eat it well.

2. 많이 먹을게. (Informal; same as the first one but means 'I will eat a lot' instead of 'I will eat it well'.) = I will eat a lot.

3. 맛있게 먹을게. (Informal; same as the first one but means 'I will eat it deliciously' instead of 'I will eat it well'.) = I will eat it deliciously.

4. 맛있게 먹겠습니다. (Formal; to your boss, people you are not comfortable with, inlaws, etc) = I will eat it deliciously.

Second, when you offer a meal to someone.

1. 맛있게 먹어. (Informal; to your friends, colleagues, people who are younger than you, etc)

2. 많이 먹어. (Informal; to your friends, colleagues, people who are younger than you, etc)

3. 맛있게 드세요. (Formal; to your inlaws, parents, neighbors, not close colleagues, etc)

4. 많이 드세요. (Formal; to your inlaws, parents, neighbors, not close colleagues, etc)

5. 맛있게/많이 먹으렴. (Friend's mom's way to speak.)

6.  맛있게 드십시오. (Extremely formal; to your strict boss)

Note:

• 맛있게 = deliciously
• 먹다 = to eat
• 드세요 = to eat (formal)
• 많이 = a lot / much / many
• ~할게 = I will do ~