It's important to note that "estar de novio" emphasizes the committed nature of the relationship and is usually used to describe a serious, long-term relationship.
Here are some examples:
* "He's been 'estar de novio' with his girlfriend for two years." -> He's been in a relationship with his girlfriend for two years.
* "They are 'estar de novio' and planning to get married soon." -> They are dating and planning to get married soon.
While "being in a relationship" is the most accurate translation, you can also use "to be going steady" or "to be sweet on someone" depending on the context.