R: Problems when calling a function within mutate()

Question

I have a function to calculate a frequency value for specific strings. Unfortunately, I failed to apply this function within mutate():

# Create a data.frame with some sample texts
df <- data.frame(var1 = c("one short text", "another example", "something", "agsjehjr", "ajase"), 
language = c("D", "D", "I", "I", "D"))

# Create a lookup-table with values for two-character-combinations 
lookup_table <- data.frame(token = c("ex", "te", "th", "id"), 
                           value = c(0.2, 0.2, 0.3, 0.25),
                           language = c("D", "D", "I", "I"))

# A function to calculate a value for a specific word
lookup_text <- function(text, language = "D"){

  df <- data.frame(token = tokens_char(text, window = 2)) %>%
    mutate(language = language) %>%
    left_join(lookup_table, by = c("token", "language")) %>% 
    mutate(value = ifelse(is.na(value), 0, value))
  
  return(mean(df$value, na.rm=TRUE))
    
}

# Check: This works as expected
lookup_text("example", "D")

# This doesn't work, what am I doing wrong?
df %>% 
  mutate(new_var = lookup_text(var1, language = "D"))

Unfortunately, I get the same value for all texts. There's a warning message:

Warning messages:
1: Problem while computing `new_var = lookup_text(var1, language = "D")`.
i the condition has length > 1 and only the first element will be used 
2: Problem while computing `new_var = lookup_text(var1, language = "D")`.
i first element used of 'length.out' argument 
3: Problem while computing `new_var = lookup_text(var1, language = "D")`.
i longer object length is not a multiple of shorter object length