Support for single-file JS translation files in loco-translate plugin, includes a patch of the plugin to allow hijacking the file generation process.

i18n.php

<?php

/**
 * Register actions and filters
 */
function bootstrap() : void {
  // Setup script translations
  add_action( 'wp_enqueue_scripts', __NAMESPACE__ . '\setup_script_translations' );
  
  // Filter the script translation lookup process to avoid linking to relative path names
  add_filter( 'load_script_translation_file', __NAMESPACE__ . '\filter_script_translation_path', 10, 3 );

  // Fix loco-translate JSON file generation in v2.5 (uses patched/injected filter)
	add_filter( 'pre_loco_write_json', __NAMESPACE__ . '\\loco_generate_json_translation_file', 10, 4 );
}

/**
 * Load JSON translation files for JS
 *
 * @return void
 */
function setup_script_translations() : void {
	wp_set_script_translations( 'YOUR_SCRIPT_HANDLE', 'YOUR_DOMAIN', WP_CONTENT_DIR . '/PATH/TO/YOUR/LANGUAGES/FOLDER' );
}

/**
 * Filter script translation path to use uploads folder
 *
 * @param string $file
 * @param string $handle
 * @param string $domain
 *
 * @return void
 */
function filter_script_translation_path( string $file, string $handle, string $domain ) : string {
	$locale = determine_locale();
	if ( $domain === 'YOUR_DOMAIN' ) {
		$file = WP_CONTENT_DIR . '/PATH/TO/YOUR/LANGUAGES/FOLDER/YOUR_DOMAIN-' . $locale . '.json';
	}

	return $file;
}


/**
 * Generate JSON translations of a bundle in a single file without hashes
 *
 * This fixes an issue with the way WordPress / Loco generates files, requiring exact file paths on
 * production as during the generation process, which is broken by any build/concatenation process like webpack.
 *
 * @param \Loco_gettext_Data $po
 * @param \Loco_package_Project $project
 * @param \Loco_gettext_Compiler $compiler
 *
 * @see \Loco_gettext_Compiler::writeAll   for the injected action to trigger this (loco_written_compiled_translations)
 * @see \Loco_gettext_Compiler::writeJson  for the JSON file writing process
 * @see \Loco_gettext_Data::msgjed         for the formatting process of the JSON file
 *
 * @action loco_written_compiled_translations
 *
 * @return void
 */
function loco_generate_json_translation_file( $result = null, \Loco_package_Project $project = null, \Loco_gettext_Data $po, \Loco_fs_File $pofile ) {
	// :shrug:
	if ( empty( $project ) ) {
		return;
	}

	$domain = $project->getDomain()->getName();

	$name = $pofile->filename() . '.json';
	$jsonfile = $pofile->cloneBasename( $name );

	// 1. Collect all JS translations from the PO file
	$data = [];
	foreach ( $po->exportRefs( '\\.js' ) as $ref => $fragment ) {
		$data = array_merge( $data, $fragment->exportJed() );
	}

	// 2. Get headers from PO file
	$head = $po->getHeaders();

	// 3. Format the file content in JSON
	$formatted = json_encode( [
		'translation-revision-date' => $head['PO-Revision-Date'],
		'generator' => $head['X-Generator'],
		'source' => '',
		'domain' => $domain,
		'locale_data' => [
			$domain => $data,
		],
	] );

	// 4. Save the JSON file with the same name of the PO file but with .json extension
	try {
		$fs = new \Loco_api_WordPressFileSystem;
		$fs->authorizeSave( $jsonfile );
		$jsonfile->putContents( $formatted );
	} catch ( \Loco_error_WriteException $e ) {
		\Loco_error_AdminNotices::debug( $e->getMessage() );
		\Loco_error_AdminNotices::warn( sprintf( __( 'JSON compilation failed for %s', 'loco-translate' ), $ref ) );
	}

	$jsons = new \Loco_fs_FileList;
	$jsons->add( $jsonfile );
	return $jsons;
}

loco-translation-json-generation-2.5.patch

This patch for loco-translate v2.5 adds a filter to shortcircuit the JSON files generation process of loco-translate.
Use it with vaimo/composer-patches to apply it to the plugin via composer, or apply the changes manually otherwise.

@package wpackagist-plugin/loco-translate

diff -ru ./src/gettext/Compiler.php ./src/gettext/Compiler.php
--- ./src/gettext/Compiler.php 2021-02-12 00:49:38.000000000 +0200
+++ ./src/gettext/Compiler.php 2021-02-12 00:53:49.000000000 +0200
@@ -102,6 +102,19 @@
         $pofile = $this->files->getSource();
         $base_dir = $project->getBundle()->getDirectoryPath();
         $domain = $project->getDomain()->getName();
+        /**
+         * Filter to allow external handling of JSON file generation
+         *
+         * @param \Loco_gettext_Data $po
+         * @param \Loco_package_Project $project
+         * @param \Loco_gettext_Compiler $compiler
+         *
+         * @return Loco_fs_FileList|null
+         */
+        $pre = apply_filters('pre_loco_write_json',null,$project,$po,$pofile);
+        if( $pre !== null) {
+            return $pre;
+        }
         /* @var Loco_gettext_Data $fragment */
         foreach( $po->exportRefs('\\.js') as $ref => $fragment ){
             // Reference could be source js, or minified version.