There are number of ways you can contribute to the project.
Traccar project contains server side part, web interface and mobile apps. If you want you can contribute code to any of those projects. Documentation might not be perfect, but the source code is very clean and pleasant to work with.
Translations are important to many people who don't know English well enough or prefer interface their native language. If you want to contribute to project localization, you can do it through Transifex Traccar project.
If you don't have time to contribute to the project but you want to help, you can also support it by donating.