The interviewer is gauging your willingness and capacity to learn new technologies and languages on the job to measure how fast you will be brought up to speed and your long term value to the company.
In most (if not all) software/technology settings, no two software will look the same. They'll be designed with different (if not mixed) conventions, patterns, architectures, and languages, including domain-specific-languages. As a result, when exposed to the software, you will be exposed to a learning curve. This question gauges your ability and willingness to learn the technologies or languages compared to other candidates. Candidates who answer this question well (i.e. with enthusiasm, perseverance, or a capacity for learning quickly) depict a realizable edge against other candidates. (Assuming they're not lying) said candidates will take less time to be brought up to speed by current teams, thereby increasing the overall efficiency and investment value once the candidate begins submitting features and bug-fixes.
Long Term Value
Similarly, it's also a gauge at your long term value to The Company. As a Developer, it's important for you to be capable of learning new technologies, languages, libraries, etc. This enables The Company and yourself to leverage new technologies rapidly. Without this capacity, the software may grow stale and lose competitiveness against competing products who leverage new technologies.