Add preferred language and country field retrieval and editability

[teolemon]

What - Proposed solution

Add preferred language and country field retrieval and editability

https://world.openfoodfacts.org/cgi/user.pl?type=edit&userid=monsieurtanuki
https://world.openfoodfacts.org/cgi/user.pl?type=edit&userid=teolemon

<select name="preferred_language" id="preferred_language-select">
                                <option value=""></option>
                                
                                    <option value="ab"> Abkhaz</option>
                                
                                 <select name="country" id="country-select">
                                <option value=""></option>
                                
                                    <option value="en:afghanistan"> Afghanistan</option>
                                
    ```
                              

[monsieurtanuki]

@teolemon We'll probably be stuck like in the "delete" account case: the user needs to be connected in order to change the personal preferences.
And so far in dart we don't really "connect".
cf. #746 openfoodfacts/smooth-app#4162 openfoodfacts/openfoodfacts-server#8940