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Which language would be best for building billing software.
Since all languages are almost on par, your answer is the language you are most familiar with.
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I want to build a billing software for my college project and maybe I would also try to sell them.
I want to add features like stock management and barcode printing also.
Beware, a billing software is not general purpose, it is usually optimized for the usage. A billing software for burgers is not the same as for pizzas.
Advice: make your software modular, define what will do the core function.
If it is done before deadline, make the stock management and barcode in a way that it can be hidden and deactivated if not finished on deadline. So you can show a polished software, even if not your full project.