It depends on wether this person comes from a countrythat speaks several languages or not. Finland was bilungual (Swedish and Finnish) and the state apparaturs required people to be bilingual. Swedish was taught at school and while often quickly lost due to no usage, some could at least understand Swedish. About 5% of the population had Swedish as their native tounge.

In Sweden, a secondary language was required to pass "studentexamen" (2-4 years of gymnasium between the age of 15 and 18). 1900 about 1% of the population passed. In 1935 it was about 7%. German was the most common secondary language until 1944, when it was replaced by English. However, French was also common.

In nations with intermingled people, for example Poland (Poles, Germans, Ukranians, Ruthenians and Lithiuanians), Czechoslovakia (Czechs, Slovaks, Germans, Ruthenians and Hungarians), Belgium (Germans, Walloons and Flemish) and Yugoslavia (Serbs, Montenegrins, Albanians, Croats, Slovenes, Hungarians, Bulgarians) speaking more than one language was common.

Dutch people had (and still have) compulsory lessons in German, French and English. French was the main language taught in schools before WWII, German took over during the 20's as the preferred trade language, English became the most spoken trade language after WWII.

Any Dutch person in 1939 who had attended either a trade school or a gymnasium (highest-level secondary education; the Dutch secondary school system is tiered according to scholastic aptitude) would have been able to read German, French and English with considerable skill. However, this does not mean people spoke it fluently. Students of the gymnasium would also have been able to read, and have brief conversations in, Latin and ancient Greek.

Source in Dutch: http://www.talenexpo.nl/hoofdstuk.php?mnu=16 (Dutch Society for Living Languages)

I don't know if you'd account for dialects and regional languages as a second language but methinks one should.

Certainly in 1939 some of my family only spook Walloon at home and French at school, others in Belgium would speak Dutch at home and French at school, if not latin and greek. I guess there were also more bilingual Welsh-English, Catalan/Castillian, Basque/Castillian, Breton/French, Irish/English ... speakers. I am sure the decline of local languages and dialects started earlier. But europe was very rich in those.