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When decreasing the number of the steps in the user flow affects the app/website usability? is there any reference ?
in the following example, the change language appears in a separate screen is it good ? or because there is only two options merge them in one screen ?
Generally speaking: Yes. Using less steps to get to the desired goal is a good thing. However, this is not the case if the complexity of the steps rises instead (see for example the performance difference between one-step and multi-step checkouts). So: If reducing steps DOES mean also reducing complexity - then yes, it is good. If not, then not.
Concerning your language switch question: "Standard" would be to offer a list of choosable items, not using buttons like you did, but use the button space for confirmation/cancel action instead. If you intend to add another question, your concept breaks out of bounds - and you also might like to consider wording length.
So yes, you for sure could use On-Off switches or checkmarks directly in the list, but this is not their intended functionality, since these are made for yes/no or on/off situations. Your situation is more similar to a single-select radio button style choice. For not confusing the user by breaking out of standard AND because two steps will make the interface less cluttered and thus less complex, I'd go for the 2 step approach: Click on "switch language", then select one from a list.
Reduce the cognitive load for the steps, design for speed. If the user advances fast and with a small cognitive load, focusing on a specific task, an additional step or two should not be a barrier or a reason for abandonment.
Contextual help is important for customers to understand steps with implicit cognitive load
In this case (Language selection) I would pull out the native selector with both languages, they select and it gets done. Simple. That type of modal dialog is normally used for confirmation, not so much for selection. Also that dialog does not scale. If you add a third language you are dead in the water. While a native selector can scale to several more languages.
Always look for UI solutions provided by the phone OS for these type of simple known interactions. You can benchmark how other apps do it too, that way you can gain an insight to what users are used to do in this specific use case.
