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Instead use the feature to Implement a custom translate5 skin / look & feel and put your custom phtml help page in the client-specific folder, where it will not be overwritten.

documentationUrl configuration definition

The same way as the loaderUrl, in the editor overview if you define an url in this config runtimeOptions.frontend.helpWindow.editor.documentationUrl 

the content will be rendered in additional tab in the help window. By default the documentation content is rendered from editordocumentation.phtml (can be overwritten with client-specific overwrite) located in the help scripts folder defined above. You can also define remote url which content will be loaded in iframe inside the window. If configured, the help window can contain 2 tabs: "Videos" and "PDF documentation". The "Videos" tab is meant for displaying video content documentation and the "PDF documentation" for showing written documentation. If you provide placeholder ( {0} ) in the url ex: http://translate5.local/editor?lang={0} , the language locale of the current user will be set when the url is called.

Help page localization

The current locale of the authenticated user can also be passed to the loaderUrl via the optional placeholder "{1}". This fragment is replaced with the current users locale.

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