Json – i18n error message: $ operator is invalid for atomic vectors

I try to make a shiny dashboard with translations. I got the example from the Appsilon i18n hp.

Here is the JSON:

"languages": ["en", "pl", "hr"],
"translation": [
    {
        "en": "MtCars Dataset Explorer",
        "pl": "Eksplorator danych: MtCars",
        "hr": "Istraživanje MtCars Dataseta"
    },
    {
        "en": "Change language",
        "pl": "Wybierz język",
        "hr": "Promijeni jezik"
    },
    {
        "en": "Select",
        "pl": "Wybierz",
        "hr": "Odaberi"
    },
    {
        "en": "This is description of the plot.",
        "pl": "To jest opis obrazka.",
        "hr": "Ovo je opis grafikona."
    },
    {
        "en": "Scatter plot",
        "pl": "Wykres punktowy",
        "hr": "Dijagram raspršenja"
    },
    {
        "en": "X-Axis",
        "pl": "Oś X",
        "hr": "X os"
    },
    {
        "en": "Y-Axis",
        "pl": "Oś Y",
        "hr": "Y os"
    }
]

Here is the R code:

## load libraries
library(ggplot2)
library(ggpubr)
library(extrafont)
library(shiny)
library(shinydashboard)
library(shiny.i18n)

## UI translation
i18n <- Translator$new(translation_json_path = "./trans1.json") # load translation file
i18n$set_translation_language("en") # set default language

ui <- fluidPage(
  shiny.i18n::usei18n(i18n),
  tags$div(
    style='float: right;',
    selectInput(
      inputId='selected_language',
      label=i18n$t('Change language'),
      choices = i18n$get_languages(),
      selected = i18n$get_key_translation()
    )
  ),
  titlePanel(i18n$t('MtCars Dataset Explorer'), windowTitle=NULL),
  sidebarLayout(
    sidebarPanel(
      width=3,
      tags$h4(i18n$t('Select')),
      varSelectInput(
        inputId='x_select', 
        label=i18n$t('X-Axis'), 
        data=mtcars
      ),
      varSelectInput(
        inputId='y_select', 
        label=i18n$t('Y-Axis'),
        data=mtcars
      )
    ),
    mainPanel(
      plotOutput(outputId='scatter'),
      tags$p(i18n$t('This is description of the plot.'))
    )
  )
)

server <- function(input, output, session) {
  observeEvent(input$selected_language, {
    update_lang(session, input$selected_language)
  })
  
  output$scatter <- renderPlot({
    col1 <- sym(input$x_select)
    col2 <- sym(input$y_select)
    
    ggplot(mtcars, aes(x= !! col1, y= !! col2)) + 
      geom_point(size=6) + 
      ggtitle(i18n$t('Scatter plot'))
  })
}

app <- shinyApp(ui=ui, server=server)

Start the app with runApp(app) and I get the error message

Warning: Error in $: $ operator is invalid for atomic vectors

I only use "$" on the "input" and "i18n" variables. If I ask R:

is.atomic(i18n)
[1] FALSE

If I launch the code without the 7 selectInput lines, I do not get the error message.

What did I do wrong? What do I not get? Have there been changes in packages that I missed?

Thank you for your help!