With a global customer base you need to be sure that you efficiently make or take payments from your customers without causing them inconvenience.
You'll need to add a gem source if you don't already have it.
The International Bank Account Number (IBAN) is attached to all accounts in the EU countries, the EEA countries (Norway, Iceland and Liechtenstein) and most of the remaining European countries.
It's poorly architected - there is a class but it is not namespaced so it cannot be autoloaded. Move the four initial characters to the end of the string 4.
All methods are delegating to procedural functions. Replace the letters in the string with digits, expanding the string as necessary, such that A or a = 10, B or b = 11, and Z or z = 35. Calculate mod-97 of the new number, which results in the remainder 7.
Bank account data changes over time, with many changes being created by the implementation of the Single Euro Payments Area (SEPA).