Whether you're studying language and its application to the law, or a law student studying foreign language and foreign law, time in a law school setting is required. If you are a law major who is doing a secondary focus on modern languages then you will go through the entirety of law school, or the entirety of the law requirements for a bachelor's degree. If on the other hand you're focusing on modern language and its legal applications you will only do a single year in law school.
Language and law are both products of a given culture and history. In order to properly understand the language of another country, you need to understand how that country came to be what it currently is. This is why there are concepts or thoughts that don't translate because they deal with a unique cultural phenomenon. As such, courses for both foreign law, and for foreign language, deal with not only what a culture is now, but how it came to be that way over time.
Both modern language and foreign law majors will require that students spend time abroad in the country or countries that they're focusing on. The requirements will vary depending on level of study and degree, and it could be anywhere from a single semester to a year in that foreign cultural setting. This is meant to give the student a real feeling for what goes on in this culture and to integrate them into the organic world of language and ideas, rather than those things being dry concepts in a book.
A large portion of a student's studies in modern languages and law focus on learning the foreign language that's being focused on. However, if a student can prove through testing that he's already proficient at a certain level in that foreign language --- such as a student who speaks and reads fluent Spanish and English --- then that student may be exempt from many of the foreign language courses that would otherwise be required.