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What is a good modern language to use for a Business Rules project? 

This is a rather vague question, but I'm trying to solve a specific problem and I'm inexperienced in most of the potential solutions, so please forgive the inherent ambiguity. 

I have access to a database with millions of historical financial transactions, with new transactions being added every day. Based on these transactions I want to apply several hundreds of business rules. These could be things like, "apply sales tax per lookup table based on city and state," or "record commissions based on the internal salesperson and their commission rate." It could also include things like, "bill client X twice per month between April and October at rate R1 for type A transactions and rate R2 for type B transactions, and a flat $100 per month from November to March." 

Eventually I'd like for the business rules to be programmable via a web interface, but that's unnecessary at the beginning. My main concern is creating a system that is reliable and flexible enough to handle this level of complexity while still being relatively maintainable and comprehensible. 

Unfortunately, it seems that COBOL is one of the most popular languages for this kind of problem, but I'd rather not go that route. I've used R and SQL at much smaller scale in a similar problem space, but it wasn't ideal. I'm familiar in passing with BREs like Drools, but have never actually worked with them, and my general perception is that they are often difficult to maintain. 

I don't mind learning a new stack for this project; Go, python, or Rust have all caught my eye in the past. But I keep wondering if I'm missing something that has more inherent support for this use case.