When I used to be involved in hiring at my company, I often saw people clarify their level of expertise in various programming languages; for example:
- Fluent: Java, C, Python, Lua, ...
- Familiar: R, C#, ...
Languages: Java (fluent), Python (fluent), R, C#
Languages: Java (expert), Python (intermediate), Perl (intermediate), C# (beginner)
Along the lines of the examples above, you can list your new language as "in-training" or similar, or list it under your hobbies or interests. Don't try to pass it off as a de-facto skill if you aren't very proficient yet. It's true that many programming concepts are language-agnostic but there is a learning curve associated with using a new language.
The employer wants to hire you for what you already know, and if you lead the employer to believe you could jump into the middle of working on a project using that language, you may find yourself in over your head. At the very least, qualify your level of experience so the employer knows that you've dabbled but may not be very productive in that language.
Regardless, you should also list some of your past projects and what languages, frameworks, etc., you applied while working on those projects. If you're working on a substantial enough side project as you learn a new language, it would be beneficial to list that project, as well.