Thank you.
The default bamburgh font doesn't support French accents. It works if I import Bamburgh.ttf:
theme_settings {
font = userfont_bamburgh
}
fonts {
userfont_bamburgh : base64_ttf horz_pixels = "8" vert_pixels = "8" font_scale_multiplier = "1.0" experimental_line_height_ratio = "1" snap_vert = "true"
I also had to change horz_pixels = "8" vert_pixels = "8" to horz_pixels = "6" vert_pixels = "12" as you suggest here.