Dashboard and Site Language only partially translated (in own theme)

Question

I developed my own Block Theme.

The default language of my Wordpress installation is English (US).

If I change it to e.g. German (Switzerland), only a few words/sentences are translated in the dashboard:

Another example: When using the "Login/Out Block", I see the German text "Anmelden" in the Full Site Editor, but not in the published post on the front-end (where I still see "Log in").

I stumbled upon a post where he uses a Divi theme (which I don't), mentioning that when activating a default Wordpress theme the translations work.

So out of curiosity I temporarily activated the Twenty Twenty Four theme, and everything was translated as expected.

So now I'm left with the question:

What theme setting or detail am I missing?

Answer 1

Finally I found the issue!

The following filter was causing the theme to only translate partially:

function howdy_message($translated_text, $text, $domain) {
    $new_message = str_replace('Howdy', 'Aloha', $text);
    return $new_message;
}
add_filter('gettext', 'howdy_message', 10, 3);

I replaced it with the following that now works:

function howdy_message($translated, $text, $domain) {
    if (false !== strpos($text, 'Howdy'))
        return str_replace('Howdy', 'Aloha', $text);

    return $translated;
}
add_filter('gettext', 'howdy_message', 10, 3);

Why, I don’t know, but by stripping down my function.php bit by bit I managed to find the function that was causing the issue.