Hi
1. At the moment we do not support filters from fields other than custom fields , categories and tags.
Though it is in our candidate features list, to support filters from native fields like the author and the created/published date, in the future.
So your solution, could be to create a custom field of type calendar for that. Going further, you can create a php script that updates that custom field's values automatically, reading the value from your created date or another one.
2. No it is not a bug. Custom fields are multi-lingual in their core.
I understand using the same custom field in more than 1 language when the values are universal (e.g. numbers, sizes).
But when the values change in each language, you are advised to use different custom fields in each language.
In any case, if you would like to achieve that, you can do it with a template override of the JFilters - Filters module.
In the file: modules/mod_jfilters_filters/tmpl/_dummy_elements.php
Check the line 100: <span class="jfilters-item__label-text"><?php echo htmlspecialchars($option->getLabel()); ?></span><?php
It could be :
<span class="jfilters-item__label-text"><?php echo htmlspecialchars(\Joomla\CMS\Language\Text::_($option->getLabel())); ?></span><?php
* I provide that as a helping hint, rather as a final working solution. Our support does not provide modifications officially.
Also as explained, i am not in favor of that implementation.
Kind regards,
Sakis Terzis
Founder and Lead Developer