。
She's married to a foreigner.
Literal
She [topic-は] foreigner [with-と] married-is.
結婚する is a punctual verb — the act of getting married happens at a moment in time — so 結婚した = 'got married' (the event). Add ~ている and you switch to the resulting state: 結婚している = 'is married' (the ongoing condition). This is the same pattern that gives 死ぬ → 死んでいる ('is dead,' not 'is dying') and 知る → 知っている ('know'). The kanji compound 国際結婚 is the standard label for a Japan–non-Japan marriage; once a remarkable thing socially, these have become considerably more common in recent decades, though the term itself still carries category-naming weight in everyday talk.