Whenever the thing referred to is a human, it is more formal to use the relative pronoun who.
Expatriates who are new to a country will have much to learn.
Using that, while not incorrect, is somewhat less formal.
Expatriates that are new to a country will have much to learn.
In certain circumstance, who can be used instead of that, if the stylistic purpose is to anthropomorphize the referrent.
Firms who manage to attract high-level workers are undervalued be the market.