Template : how to get first character of the title?

The ultimate goal is to be able, in the node--my-content-type.html.twig, to use this title in a statement using an article in french, language in which the article depends on the first letter of a word:

• Banane => de Banane;
• Orange => d'Orange;

So based on Twig documentation, I'm trying to use:

• {% if label|first|lower == o %}
  OR
• {% if label|slice(0, 1)|lower == o %}
  OR
• {% if label[:1]|lower == o %}

But none of it worked so far.