Hi, I see there are ‘languages’ folders in every add-on, for example: motopress-hotel-booking > languages mphb-request-payment > languages mphb-notifier > languages etc.
Also, in wp-content we have ‘languages > plugins > ‘file.mo/.po’
When we place our adjusted translation files in the standard WP structure, the translations do not work. Is there no support for the standard WP structure?
The standard WordPress structure for language files overrides works fine, you should make sure override files are named properly. Try using Loco Translate plugin, which automatically places language files to desired location, including /wp-content/languages/plugins/ directory. This will make override files created automatically with correct names, so you could perform further changes to them either manually or via the plugin.