I believe there are some agreements made by IATA regarding use of languages.
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Maybe the rule that many airlines apply is the correct IATA recommendation.
(1) National language of airline
(2) Language of country, airport of departure
(3) Language of country, airport of destination
(4) English (to please some of you)
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As an example, Varig used Portuguese, English and Japanese on their flights from Sao Paulo to Los Angeles to Tokyo. Some airlines have a "language roster" and have F/As who speak certain languages generally assigned for certain routes, to cover all likely languages to be used or needed. With my airline, 12 flight attendants in the 747, we can do many languages on PA. As pilot, I always do a PA in Spanish, then English, and I might do some French, Portuguese, Dutch or German to impress the monolingual Anglo-Saxons on board...
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Happy contrails