[10.x] Handle missing translation strings using callback

[DeanWunder]

This change adds the ability to customise the behaviour of how missing translations are handled. The current behaviour is to return the key unchanged. For example {{ __('user.name') }} will render as user.name if the corresponding translation key doesn't exist in language files. This change allows a callback to define the missing translation behaviour increasing customisability. The callback can return a string which will then be returned by the get method.

An example of a use case for this is: I would like to add the ability to trigger a log message if a translation key is missed in production to detect when cryptic text such as user.name are being displayed to users in production.

There are packages to solve this problem using static analysis such as https://github.com/LarsWiegers/laravel-translations-checker, however static analysis is limited and can not pick up run-time generated keys, e.g. __("user.$field"). This change would make it possible to detect translation misses in run-time.

Example of usage in any service provider:

use Illuminate\Translation\Translator;
use Log;
// Edit: the syntax has changed since this feature was originally proposed.
// Please see docs at https://laravel.com/docs/10.x/localization#handling-missing-translation-strings
Translator::handleMissingTranslationKeyUsing(function ($key) {
    Log::error("Missing translation key [$key] detected.");
});

These changes also include functionality to prevent infinite loops where the user defined callback itself contains a missing translation.

This idea was originally proposed at: #48862

[bram-pkg]

This is a lovely feature, personally I'd like to use this to report missing translation string automatically

[Sophist-UK]

Typo - "infitite"->"infinite"

[antimech]

Suggested change

	// Call user defined missing translation handler callback, prevent infitite loops.
	// Call user defined missing translation handler callback, prevent infinite loops.

[DeanWunder]

Fixed in this PR, thank you.

[Sophist-UK]

It is a very minor procedural point, but force-pushing commits is not generally considered the way to make minor incremental changes to code in a PR - a normal push is generally used.

[Sophist-UK]

Congrats @DeanWunder for getting this PR accepted. 👍

[Sophist-UK]

Congrats again to @DeanWunder - this PR is now live in Laravel 10.33.0.

[DeanWunder]

Thanks @Sophist-UK I can now say I contributed to laravel 😍
We have now deployed this to our websites and look forward to the benefits it will provide!