GeoContent.php

<?php

class GeoContent extends DataObject
{

    protected static $db = [
        'Code'    => 'Varchar',
        'Content' => 'HTMLText'
    ];

    protected static $has_one = [
        'Page' => 'Page'
    ];

}

Page.php

<?php

class Page extends SiteTree
{

    private static $db = [
        'LeadText'         => 'Text',
    ];

    private static $has_one = [];

    private static $has_many = [
        'GeoContentBlocks' => 'GeoContent'
    ];

    public function getCMSFields()
    {
        $fields = parent::getCMSFields();

        $fields->insertAfter(new Tab('GeoContent'), 'Main');

        $fields->dataFieldByName('Content')->setTitle('Default content - United States');
        $fields->insertAfter($fields->dataFieldByName('SecondaryContent'), 'Content');

        foreach (EcommerceCountry::list_of_allowed_entries_for_dropdown() as $code => $title) {
            if ($code === 'US') {
                continue;
            }
            $geoContentBlock = $this->GeoContent($code);
            /** @var HtmlEditorField $field */
            $field = HtmlEditorField::create("Geocontent[{$code}]", "Content for {$title}");
            if (is_a($geoContentBlock, 'GeoContent')) {
                $field->setValue($geoContentBlock->Content);
            }
            $fields->addFieldToTab('Root.GeoContent', $field);

        }

        return $fields;
    }

    public function GeoContent($country = null, $createIfNotExists = false)
    {
        if ($country === null) {
            $country = EcommerceCountry::get_country();
        }

        $content = $this->GeoContentBlocks()->filter('Code', $country)->first();
        if ((!$content || !$content->exists()) && !$createIfNotExists) {
            return $this;
        }

        if ((!$content || !$content->exists()) && $createIfNotExists) {
            /** @var GeoContent $content */
            $content         = GeoContent::create();
            $content->Code   = $country;
            $content->PageID = $this->ID;
            $content->write();
        }

        return $content;
    }

    protected function onBeforeWrite()
    {
        parent::onBeforeWrite();
        foreach ($this->record as $key => $value) {
            if (preg_match('/Geocontent\[(.+)\]/', $key, $matches)) {
                if (!$value) {
                    $block = $this->GeoContentBlocks()->filter('Code', $matches[1])->first();
                    if ($block && $block->exists()) {
                        $block->delete();
                    }
                } else {
                    $block          = $this->GeoContent($matches[1], true);
                    $block->Content = $value;
                    $block->write();
                }
            }
        }
    }

}