Bug 7282 - invalid language selection

Bug 7282 - invalid language selection

- ensure that without cookie, language selection is based on browser
  preferences
- refactor function to obtain langugage for stem in search (opac + intranet)

Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
To reproduce the bug:

- Activate en and fr-FR for OPAC
- Clear your browser cookies
- Select your language preference in your browser: fr, fr-fr, en
- Load OPAC main page
- OPAC is displayed in English, rather than French as asked by browser
  preferences

  Apply the patch and test:

  - Clear your browser cookies
  - Load OPAC main page
  - Pages are displayed in French

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
(cherry picked from commit 0ad7e76cb7cebf16310a95377f88047f09b66d24)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>