I wrote a short script which searches all language fields and call an update statement to copy the content to another language
<?php
include(__DIR__."/index.php");
$field_db_names = array();
foreach($wire->wire('fields')->find("type=FieldtypeTextareaLanguage|FieldtypeTextLanguage") as $key => $field) {
$field_db_names[] = strtolower($field->name);
}
$languageMap = array(
'data' => 'data1017'
);
$db = $wire->wire('database');
foreach ($field_db_names as $field) {
echo "processing field ".$field."\n";
foreach ($languageMap as $from => $to) {
echo "copy $from to $to \n";
$sql = sprintf("UPDATE field_%s SET %s = %s WHERE %s = '';",$field,$to,$from,$to);
$db->query($sql);
}
}
$languageMap contains the information what field is used to fetch the content. so in this example data is copied to data1017 if it is still empty (so nothing is overwritten).