I'd like to render the following date...

Lundi 3 décembre 2018

...but I keep getting a lower case first letter.

Here's the relevant piece of code:

{{ date_group format_localized="%A %d %B %Y" | ucfirst }}

Any ideas as to what I could do to fix this?

system.yaml is set as follows:

locales:
  fr:
    full: fr_CH.utf8
    name: Français

Thanks!