It's hard to say exactly without knowing what the external file looks like, but I'll try to make it generic enough.
func load_file(file_path):
var file = File.new()
file.open(file_path, file.READ)
var text = file.get_as_text()
return text
func get_random_word_from_file(file_path):
var text = load_file(file_path).strip_edges()
var words = text.split(" ")
for i in range(words.size()):
# here you can add whatever you want to remove
# any unwanted characters
words[i] = words[i].replace(",", "")
words[i] = words[i].replace(".", "")
print(words)
return words[randi() % words.size()]
Then call it like:
var random_word = get_random_word_from_file("res://path/to/file.txt")
$RichTextLabel.text = random_word
# or if you're using bbcode
$RichTextLabel.bbcode_text = "[center]" + random_word + "[\center]"