spring-hateoas-ext/src/main/java/de/escalon/hypermedia/affordance/Affordance.java
public void addHreflang(String hreflang) {
        Assert.hasLength(hreflang);
        linkParams.add(HREFLANG.paramName, hreflang);
    }
public void addHreflang(String hreflang) {
        Assert.hasLength(hreflang);
        linkParams.add(HREFLANG.paramName, hreflang);
    }
The "hreflang" parameter, when present, is a hint indicating what the language of the result of dereferencing the
link should be.  Note that this is only a hint; for example, it does not override the Content-Language header of
a HTTP response obtained by actually following the link.  Multiple "hreflang" parameters on a single link- value
indicate that multiple languages are available from the indicated resource.

@param hreflang to add