Moving to job advice section.
The general advice is to do what you want, follow your own passions. That will apply to many of your questions.
> I am practicing a lot of C++ and Java at the moment. However I heard that focusing on one language sounds better in a CV.
Some jobs require in-depth knowledge of a single language. Some jobs require a working with multiple languages. In general it is good for programmers to get exposed to around one new programming language every year. Do whatever you are passionate about.
If you are more passionate about general gameplay code, then follow that route. If you are passionate about some specialties, then follow those.
Do it because it is your passion, not to please some potential future employer.
> Should I practice the language that is more popular on the industry?
There is no single language with that distinction. Working on a web game? You'll need JS and HTML. Working on a console game? C++. Working on an iOS game? That's Objective C.
Learn the languages that work best for you, that you are most interested in. Follow your own passions. If you discover you love a language, learn more of it. If you don't like a language, learn what you can from it then follow your passions elsewhere.
> Next would be new languages like Rust, are they worth appearing in a CV?
Some jobs will use the language. If you are interested in learning it, then follow your passions and so so.
If you are thinking of making a big block titled "Programming Languages I Have Used" in your CV, that's ugly and not helpful to employers. When listing individual projects list the languages used as part of the project details.
Do it because it is your passion, not to please some potential future employer.
> What would/do you expect (qualities, skills, ..) from a programmer that you would want to hire?
Every person is different. One person may be a generalist with no specialties. Another person may be a specialist on physics engines and spatial optimizations. Another may be specialized in UI work, or graphics pipelines, or data tools, or build scripts.
I expect a programmer I hire will be able to do the job, whatever job I need. For universal qualities, see your next question.
> Are there certain points for what you would look in the CV? If yes, what especially?
I expect that every programmer, no matter the role, will demonstrate that they are smart and that they have passion for creating great software.
This is something that shows up everywhere in their personality, so no matter what you put on your CV it will still show through during interviews, and it will still show throw when you appear on the job.
> Would programmed games be worth anything in the software development?
Your CV is what you have done. Do what you are passionate about.
If you have done it, yes, it is useful on your CV since it shows you have done the job.
Do it because it is your passion, not to please some potential future employer.
> Would you want to know if the programmer used SFML/LibGDX, none or or even a whole engine?
I want to know what the person has done in the past, whatever that is.
Everyone's background is different. If the job happens to use one specific technology and you mention having used that technology, then I'll make a note of it.
Whatever you include, I'll quiz you on the subject in depth to learn exactly how much you know, or don't know, about the tech.
> Is contributing to open source projects mentionable? Should it specified?
If you have done it, and you think it is relevant to the job, then include it.
Do it because it is your passion, not to please some potential future employer.
> What projects would you like to see within a CV?
Show me the projects you have done in your life. The truth. Nothing more, nothing less.
Don't try to inflate things, don't include things just because you think the employer would like it. That is a disservice to both. The employer will discover quickly enough that you don't have those skills, and you will fail at the job because you don't have the skills. Both sides want success, but if you are trying to focus on what you think the employer wants and skew things because of that, both sides will get failure.
> How would I know I am ready for the industry?
For much of the world, the typical minimum HR requirement for programmers is a bachelor's degree in computer science or demonstrated past experience, meaning professionally published games.
