__() with sprintf returns untranslated string

I’m going through some legacy code and adding i18n functions throughout when I stumbled on a localisation snippet I found inside a pre_get_document_title function: (simplified)

$title="Hello World";
__( $title, 'my_text_domain' );

This returns the correct, translated string. It was recently pointed out to me however, that this isn’t valid syntax as localisation functions should only receive a string; not a variable, nor a function return value. The recommended (and correct) syntax, as seen in numerous themes:

sprintf( __( '%s', 'my_text_domain' ), $title );

Having made this change, I find that all my titles are now still in English. i.e. nothing gets translated. All the language files are still there, all I did was modify the __() calls.

Is there something I missed?

The order of the functions being called is wrong.

With this code:

sprintf( __( '%s', 'my_text_domain' ), $title );

• You are trying to translate the string ‘%s’ in the domain ‘my_text_domain’.
• you are then replacing the translated string for ‘%s’ (which is ‘%s’), by the content of the variable $title.
• Therefore your code is similar to sprintf( '%s', $title);

What you should do:

__( sprintf( '%s', $title ), 'my_text_domain' );

• sprintf() will return your title
• __() will translate the returned title

By the way, if you plan to use the title of the post only, you probably don’t need to use sprintf() but instead:

__( $title, 'my_text_domain );