_graphql-config.yml

config:
  modelConfig:
    DataObject:
      operations:
        read:
          plugins:
            fluentLocale: true
            

config.yml

SilverStripe\Core\Injector\Injector:
  SilverStripe\GraphQL\Schema\Registry:
    constructor:
      fluent: MyProject\FluentQueryPlugin
      

FluentQueryPlugin.php

<?php


namespace MyProject;


use SilverStripe\GraphQL\Schema\Field\Query;
use SilverStripe\GraphQL\Schema\Interfaces\QueryPlugin;
use SilverStripe\GraphQL\Schema\Interfaces\SchemaUpdater;
use SilverStripe\GraphQL\Schema\Schema;
use SilverStripe\GraphQL\Schema\Type\Enum;

class FluentQueryPlugin implements QueryPlugin, SchemaUpdater
{
    
    public function getIdentifier(): string
    {
        return 'fluentLocale';
    }
    
    public static function updateSchema(Schema $schema): void
    {
        $locales = Fluent::get_locales();
        $enum = Enum::create('FluentLocale', $locales, 'The locales available');

        $schema->addEnum($enum);
    }

    public function apply(Query $query, Schema $schema, array $config = []): void
    {
        $queryType = $schema->getQueryType();
        // Ensure that we only apply the Locale variable to root queries.
        // Nested queries with their own locales is super edge-casey and weird.
        if (!$queryType->getFieldByName($query->getName())) {
            return;
        }

        $query->addArg('locale', 'FluentLocale!');
        $query->addResolverMiddleware([static::class, 'setLocale']);
    }

    public static function setLocale($obj, $args, $context)
    {
        Fluent::set_locale($args['locale']);

        return $obj;
    }
}

query.graphql

query ReadPages($Locale: FluentLocale!) {
  readPages(locale: $FluentLocale) {
    nodes { title }
  }

}