Polite Phrases in Japanese

Sumimasen or gomen nasai, and the meal phrases

Sumimasen is the lighter, more public one. It gets a waiter's attention, apologises for bumping someone, and thanks a stranger who held a door.

Gomen nasai is a genuine apology to someone you know. Using it to call a waiter would sound odd, and using sumimasen after genuinely upsetting a friend would sound thin.

Itadakimasu and gochisōsama deshita bracket every meal, including one eaten alone. They are not religious; they thank everyone involved in the food reaching you.

Otsukaresama desu is the phrase with no English at all. It acknowledges that someone has been working, and it serves as hello in an office corridor, goodbye at the end of the day, and well done after a task. When in doubt in a Japanese workplace, this is the safe thing to say.