Symfony translation messages with arrays, find length - arrays

Question

Setup:
Consider the situation of a multilingual site built with Symfony 3, using YAML files for translation messages and TWIG templates for display.
In one section of content, in English, there should be an instruction showing 3 items each representing a step of a process. In German there should only be 2, while in Spanish there are 50.
The setup for this may be for example:
#messages.en.yaml
root_node:
-
title: English title 1
item: Some first list item
-
title: English title 2
item: Some second list item
-
title: English title 3
item: Some third list item
#messages.de.yaml
root_node:
-
title: German title 1
item: Some other first list item
-
title: German title 2
item: Some second German list item
#messages.es.yaml
root_node:
-
title: Spanish title 1
item: Some Spanish first list item
...
etc ...
Considering this structure, ideally, I would like the template to read the fact that root_node is an array, get it's length so that we can display the total number of steps, then create a structure in the twig template that loops over the array and outputs the steps as some translated title and item text.
Attempting to read the array length by {{'root_node'|length}} and {{('root_node'|trans)|length}} just give the length of the text though and {{root_node|length}} gives an error as the variable is not in the scope of the template, because it should be looked up through the translation service.
Question:
How can I read the length of the array at this translation key?
Should this even be attempted in this manner? If not, is there a best practice for translating arrays of unknown size (dependent on translation language) in Symfony?

Answer 1

There is no direct ability to achieve such behavior into Symfony translator. However it can be achieved indirectly by using pluralization mechanism in Symfony.
You can set custom pluralization logic by using PluralizationRules::set method and then call Translator::transChoice with passing step number as argument for choice. You will need to use transchoice(index) filter into your template to obtain required step.
You can obtain list of steps for particular locale by counting them separately from translation catalogue and passing to template. Alternatively, depending on your application's logic, you can simply obtain translated message for current step and test if it is empty string or not.