At the moment, IPA respects Accept-Language header sent by a browser.

Language is chosen according to the browser preferences. Any user can set up
their browser to choose a needed language. In Firefox, for example, this is in
Preference -> Content -> Languages.

Thus, this (language preference) is something that needs to be set in browser
and IPA would inherit the language preference set by browser. But it will
affect all IDM users and not specific ones. Also if language preference is
changed in browser all other web pages will use the same language.

RFE: allow to override browser setting by user