Register fields of custom BB modules into the WPML String Translation interface in WordPress. Add to functions.php, or custom module plugin.

wpml-beaver-build-custom-modules.php

<?php
// Translating custom Beaver Builder Modules with WPML

// Runs a filter included in the WPML String Translation plugin
add_filter( 'wpml_beaver_builder_modules_to_translate', 'wpml_modules_to_translate_filter' );

// Add each custom module to the $modules array, e.g. `custom-module-1` 
function wpml_modules_to_translate_filter( $modules ) {

  // Adding the first module.
  // `custom-module-1` would be the same as the directory name of your custom module
  // assuming you've followed BB's directions for creating modules
  $modules[ 'custom-module-1' ] = [
    'conditions' => [ 'type' => 'custom-module-1' ],
    'fields'     => [
        [
          // `field` is the same thing you would call in BB
          // when retrieving the data in $settings in your `frontend.php`
          'field'       => 'name-of-field',
          // `type` is just the label you want to be shown next
          // to the field in the String Translation Interface
          'type'        => __( 'The Field Label', 'fl-builder' ),
          // What type of field should String Translation provide?
          // As of writing, options are LINE, AREA, and VISUAL
          'editor_type' => 'LINE'
        ],
    ],
  ];
  
  return $modules;
}