What programming languages ​​to use for freelancing on foreign exchanges?

Question

Freelance

Good day!
The question is for people who are engaged in mobile development and / or freelancing on foreign and not only exchanges.
Please tell me how to enter this area and what is needed for this.
Specifically, there is no understanding of whether it is enough to know development only for android or it is necessary to know android + ios.
Or is it better to do cross-platform development, and if so, on what?
I would also like to know if the ability to write a server part is in demand (for mobile development), since this area is very interesting to me (from technologies - Spring Framework, Kotlin).
About myself - I have a little more than a year of experience as a Java EE programmer, from technologies - Java, Kotlin, Spring Framework (core, mvc, spring boot), JPA (Hibernate), JavaScript, SQL (and other work with the database).