Assuming that you are US citizen/ Resident ( Green Card ) / Resident for tax purposes and that your fiancee will be in USA ( after marriage) on a visa ( whether work or not type), her foreign income generally would not be subject to US taxes as long as you do not file MFJ. She would be a Non-Resident Alien till she either meets the Substantial Presence Test ( 183 day present counting all days in 2019, 1/3 rd. of the days present in 2018 and 1/6th. the days present in 2017 ) OR is granted a Permanent Resident Status ( Green Card ).
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