From our medical expert: In Swedish, the term “flegmone” is seldom used and has an archaic feeling. The difference as pointed out is that “flegmone” is more spread cellulitis (“cellulit” in Swedish), often becoming an abscess. “Cellulit” is a serious infection but can be rather superficial and localized, som the difference here is that “flegmone” is the same but more serious cellulitis.
To sum it up:
In Swedish, only cellulitis is used, but what we call phlegmone is more serious.