Learn English – In what context is the plural of genius, “genii” acceptable to use in a sentence

The normal plural is "geniuses"; "genii" is not used in everyday language.

The word "genius" does come from Latin, but it's gone through a long journey to get to English and its meaning has shifted quite a bit along the way. So it is pluralized according to the normal English pattern.

The issue is complicated a bit by the fact that the Latin word, with the Latinate plural "genii", does continue to be used in English for some specialist meanings.

The Oxford Dictionaries online mark the plural "genii" as being used for the following senses of the word:

a guardian spirit associated with a person, place, or institution.

as well as

The prevalent character or spirit of something such as a nation or age:
'Boucher’s paintings did not suit the austere genius of neoclassicism'

However, they indicate the usual "geniuses" for the now rather rare and archaic meaning

A person regarded as exerting a powerful influence over another for good or evil:
'he sees Adams as the man’s evil genius'
'this young man is my good genius, my guardian angel'

One last note: even when the English word is being used in one of its more common senses, "genii" might also be used as a joking plural, since using fancy Latinate plurals for ordinary words can sound funny to English speakers. I wouldn't consider that kind of joke "everyday language," though.