I have fixed the selection of a language a bit 'dirty' in the template file: /usr/lib/GNUstep/SOGo/Templates/MainUI/SOGoRootPage.wox

like this:

        <!-- LANGUAGES SELECT -->
        <div layout="row" layout-align="start end">
          <md-icon>language</md-icon>
          <md-input-container class="md-flex">
            <label><var:string label:value="choose"/></label>
            <md-select ng-model="app.creds.language">
                    <md-option value="Dutch"  ng-selected="true">Nederlands / Dutch</md-option>
                    <var:foreach list="languages" item="item">
                <md-option var:value="item">
                  <var:string value="languageText"/>
                </md-option>
              </var:foreach>
            </md-select>
          </md-input-container>
        </div>

The default setting in the /etc/sogo/sogo.conf still doesn't give me Dutch as a main language, hope some of you guys have the answer to that.

Thanks!