More information is found in the Coding Guidelines section of the “TYPO3 Explained” manual (formerly known as “Core API”).
This is just a very brief overview.
Make sure you are using the correct PHP codestyle. It is PSR-1 / PSR-2 at the time of writing and specified in the TYPO3 Coding Guidelines.
See PhpStorm setup: CGL for information on how to configure the correct coding style.
The Appendix contains information on scripts to check / fix coding guideline issues: cgl-fix-my-commit
The following rules should be applied for TypeScript: Excel Micro TypeScript Style Guide.
Language files are usually stored in a Folder Resources/Private/Language in files with the ending .xlf. While no tabs are allowed to indent in PHP files, you should edit Xliff files using tabs.