Custom labels are custom text values that can be accessed from Apex classes, Visualforce pages, or Lightning components. Custom labels can be translated into any language that Salesforce supports, and can be used to display custom messages in the user's language based on their locale settings. By using custom labels, the developer can ensure that the site is multilingual and can display the appropriate messages to the German- and Spanish-speaking customers. Custom labels can also be packaged and distributed as part of an AppExchange app. Reference: [Custom Labels], [Translate Custom Labels]

Option A is the correct SOQL query that will return a unique list of all the Contact records that have no 'Fulfilled' Orders. Option A uses a subquery to filter the Contact records based on the related Order__c records. The subquery uses the NOT IN operator to exclude the Order__c records that have the Status__c value of 'Fulfilled'. The subquery also uses the SELECT Id clause to return only the Ids of the Order__c records, which is more efficient than returning all the fields. The main query uses the DISTINCT keyword to return only the unique Contact records, and the ORDER BY clause to sort the records by the Name field. The query also uses the SELECT Id, Name clause to return only the Id and Name fields of the Contact records, which is more efficient than returning all the fields. The query also uses the LIMIT 100 clause to limit the number of records returned to 100, which is a good practice to avoid hitting the governor limits. Reference: [SOQL Subqueries], [SOQL SELECT Clause], [SOQL Functions]

The best feature to use to achieve this requirement is sharing sets. Sharing sets allow the developer to grant access to records for external users based on a common account or contact. In this case, the developer can create a sharing set that grants access to the cases for the external partners who have the same account as the support team. Role hierarchy, criteria-based sharing rules, and Apex managed sharing are not suitable for this scenario, as they are not available for external users with Customer Community Plus licenses. Reference: [Sharing Sets], [Sharing Architecture], [Trailhead: Sharing and Visibility Basics]

The best way to eliminate the limit exception error is to implement the Batchable interface. The Batchable interface allows the developer to create a class that can process large sets of data in batches, and execute logic for each batch of records. The developer can use the Database.executeBatch method to invoke the batch class, and specify the batch size and the scope of records to process. This way, the developer can avoid hitting the DML limits, as each batch of records will have its own set of governor limits. Using the @future annotation will not help, as it will only defer the execution of the method to a later time, but it will not split the records into batches, and it will still hit the DML limits. Implementing the Queueable interface will not help, as it will only allow the developer to chain asynchronous jobs, but it will not split the records into batches, and it will still hit the DML limits. Using platform events will not help, as they are used to communicate changes in data or processes across the system, but they will not split the records into batches, and they will still hit the DML limits. Reference: [Using Batch Apex], [Apex Developer Guide]

A Continuation is a way of making long-running callouts from a Visualforce page without blocking the user interface or exceeding the governor limits. A Continuation can chain multiple callouts and return the responses to the same Visualforce page. A Continuation is suitable for this scenario because the billing and tax information callouts are dependent on each other and might take up to two minutes each. An HTTP REST callout is a synchronous callout that blocks the user interface and has a limit of 120 seconds. Using an HTTP REST callout for either or both of the callouts would not meet the requirements of displaying the information at the same time and avoiding timeout errors. Reference: [Continuation Class], [Callout Limits and Limitations]