Congrats on being popular at your workplace.
I'd like some explanation of this "personal thesaurus". What exactly does the author want you to achieve with this? I know what a thesaurus is, and I hope he doesn't encourage the reader to use more flowery or abstract words for simple ones that everyone understands. From own anecdotal experience with people, most of them appreciate plain and precise language. If you want to embellish your sentences, there's a time and place for that. For example, when you know the person you're talking to is on the same page or level. If you use the wrong kind of vocabulary without being certain of the words you use, it can be off-putting.
That being said, if I understand correctly, you ask if you should use Japanese, French or Polish for your three set phrases. When do you think will that be appropriate? Probably not with the locals or your countrymen. Do you have a lot of Japanese, French or Poles in your establishment? If not, the point probably is moot. If yes, how good are you at spotting them? Imo, the answer to your question probably is no. A better way probably would be to take your stock phrases and go from there. Think "welcome and good morning" "hello, great to have you here" "thank you, have a wonderful day" etc.
If you want to be memorable, look for ways that are you. Most people either are memorable because of looks, a special feature, charisma or behaviour.

What lofivelcro said. And in my experience, smiling at people, while adressing them, is more important than whats actually said. Esp. if it's commonplace like greetings or thank yous. Even works over the phone.