Localization: Date format in header should be locale dependent

[vaclavstepan]

Describe the bug
Currently, in event header, the date is formatted using the format_event_date macro.
This macro relies on English locale custom for formatting the event date, which may not be OK for non-English languages.

To Reproduce
Steps to reproduce the behavior:

1. Go to main page of an event.
2. Switch to a different language
3. See the date format in the event header stay the same, e.g. 1-13 listopada for Polish.

Expected behavior
The date format should reflect the language, e.g. probably be set by a translation template.
For Czech it would be 12.–14. září (for example).

Additional context
The macro in question is in modules/events/templates/display/conference/_util.html

[ThiefMaster]

So what we have there is this:

    {% if start_dt.date() == end_dt.date() %}
        {{ start_dt | format_date('long') }}
    {% elif start_dt.year == end_dt.year and start_dt.month == end_dt.month %}
        {{ start_dt.day }}-{{ end_dt.day }} {{ start_dt | format_date('MMMM yyyy') }}
    {% else %}
        {% trans start=start_dt|format_date('long'), end=end_dt|format_date('long') -%}
            {{ start }} to {{ end }}
        {%- endtrans %}
    {% endif %}

I guess the MMMM yyyy should be made translatable - everything else seems to be OK since it's using locale-dependent translation? Or would you need {{ start_dt.day }}-{{ end_dt.day }} to be translatabel as well in order to add the . after the days?

[vaclavstepan]

Yes, format_date('long') is fine, only the line with start_dt.day and end_dt.day does not work well.
The
{{ start_dt.day }}-{{ end_dt.day }} {{ start_dt | format_date('MMMM yyyy') }}

has trouble that it's not using the localized form for the days - in Czech it would be 1.–2. month year, so the dots are needed.
In Czech it would work if one would take the day part from a long format, add dash, then long format for the end date.
I'm not fully sure if this is generally true/enough.

[vaclavstepan]

Similarly there are hardcoded/non-localized date formats in two files in indico/legacy/pdfinterface/latex_templates, the abstract.tex and first_page.tex. Probably all the strftime uses are possibly a trouble.