The preposition to use before "international" in English is "in." For example, "in international relations" or "in international trade." This usage is widely accepted and recognized in both formal and informal contexts. The reason for using "in" is that it indicates a field, area, or context within which something is situated or occurs. This is supported by various dictionaries and linguistic resources, such as the Oxford English Dictionary and the Cambridge Dictionary, which consistently recommend "in" as the correct preposition to use before "international.

Data from the Corpus of Contemporary American English (COCA) further confirms the prevalence of "in" before "international." The COCA is a large, annotated corpus of English that provides frequency data for words and phrases. According to COCA, the phrase "in international" appears significantly more frequently than any other preposition followed by "international." This indicates that "in" is the standard and most common choice in contemporary American English. Additionally, the American Heritage Dictionary of the English Language also supports this usage, stating that "in" is the correct preposition to use before "international" to denote a field or area of activity.