Dmitry Katsubo opened SPR-16311 and commented
As to my interpretation of ServletResponse#setLocale() JavaDoc which reads:
In the case of HTTP, the locale is communicated via the Content-Language header, the character encoding as part of the Content-Type header for text media types.
the corresponding implementation should set HttpHeaders.CONTENT_LANGUAGE which specifies the "language" of response content. Currently it sets HttpHeaders.ACCEPT_LANGUAGE, which is "language that client understands / prefers" - that one should be set by the client when it makes a request.
See also:
Affects: 5.0.1
Issue Links:
Referenced from: commits 6df1a78