In our example below, we will be adding translation to a node entity and therefore use the node namespace.
use Drupal\node\Entity\Node;
$nid = 10; // existing node id in main language.
$node = Node::load($nid); // loading node array in var
$node_trans = $node->addTranslation("FR");
//ISO lang code for translation(FR for French).
$node_trans->title = "Un titre";
$node_trans->body = [
'value' => "<p> Une paragraphe </p>",
'format' => "full_html"
];
$node_trans->status = 1; //1 for pubished state.
// adding metatag to translated content
$node_trans->field_metatag = serialize([
'title' => "tag",
'description' => "Some desc",
]);
$node->save(); // All done! translation will be created here.
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