require(kableExtra)
require(dplyr)

bold_letters = c(1,0,1)
df = cbind.data.frame("Noms" = c("A", "B", "C"), "var1" = c(1,2,3))

df %>%
  mutate(Noms = cell_spec(Noms, bold = ifelse(bold_letters==0,FALSE,TRUE))) %>%
  kable(format = "latex", escape = F, row.names = F, align = "c")

begin{tabular}{c|c}
hline
Noms & var1\
hline
<span style=" font-weight: bold;    " >A</span> & 1\
hline
<span style="     " >B</span> & 2\
hline
<span style=" font-weight: bold;    " >C</span> & 3\
hline
end{tabular}