How can I display a custom maintenance message on Magento 2.2.4

For this You need to update below file

pub/errors/default/503.phtml

and design accordingly

but best practice to override this into custom error page for this follow below steps.

Go into the pub/errors

Create a copy of the local.xml.sample and rename it as local.xml Edit the with your custom name. Ex – customerrorpage

<skin>customerrorpage</skin>

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Next create a copy of the “default” folder and rename it with your skin name. Ex – customerrorpage

Locate the 503.phtml inside the newly created folder and add your custom code. Add the styles inside css/styles.css and relevant images under images folder.

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For particular language (store view wise) you need to do using .htaccess

In you .htaccess add an rewrite rule that appends a skin parameter to your URL. Eg.

RewriteCond %{HTTP_HOST} ^french.example.com$
RewriteCond %{DOCUMENT_ROOT}/var/.maintenance.flag -f
RewriteCond %{QUERY_STRING} !(^|&)skin=french(&|$) [NC]
RewriteRule ^ %{REQUEST_URI}?skin=french[L]

Copy pub/errors/default to pub/errors/french

Change/translate template files to your needs

Reference Multistore Maintenance Mode Magento 2

Thanks

you can go to pub directory and find below path:

pub/errors/default/503.phtml

and quick edit on the 503.phtml file and edit this lines also css add here