Login.py

from tkinter import *
from tkinter import messagebox
import os

forbidden_chars = ["\\", "/", ":", "*", "<", ">", "|"]

# login
login = Tk()
login.title("Login")
login.resizable(0, 0)

Label1 = Label(text="Username: ")
Label1.grid(column="1", row="1")

u_name = StringVar(value="")
username = Entry(login, textvariable=u_name)
username.grid(column="2", row="1")

Label2 = Label(text="Password: ")
Label2.grid(column="1", row="2")

pass_var = StringVar(value="")
password = Entry(login, textvariable=pass_var, show="*")
password.grid(column="2", row="2")

info_label = Label(text="NOTE: you can't type these\n characters on username:\n \ / : * < > |", fg="gray")
info_label.grid(column="1", columnspan="2", row="3")

loginButton = Button(text="Login", command=lambda: verify())
loginButton.grid(column="1", sticky="e", row="4")

sign_inButton = Button(text="Sign in", command=lambda: sign_in())
sign_inButton.grid(column="2", sticky="w", row="4")

login.mainloop()


def verify():
    if os.path.isdir("./"+u_name.get()):
        if os.path.isfile("./"+u_name.get()+"/password.txt"):
            file = open("./"+u_name.get()+"/password.txt", "r")
            if file.read() == pass_var.get():
                def english():
                    window = Tk()
                    window.title("")
                    window.resizable(0, 0)

                    # program (english)

                    window.mainloop()

                def italian():
                    window = Tk()
                    window.title("")
                    window.resizable(0, 0)

                    # program (italian)

                    window.mainloop()

                login.destroy()

                # select language
                root = Tk()
                root.title("ROOT")
                root.maxsize(30, 120)  # to fix
                root.minsize(30, 120)  # to fix

                menu = Menu(root)
                root.config(menu=menu)
                language = Menu(menu)
                root.add_cascade(label="Select Language", menu=language)
                language.add_command(label="English", command=lambda: english())
                language.add_command(label="Italian", command=lambda: italian())

                root.mainloop()
            else:
                messagebox.showerror(title="FATAL ERROR", message="Wrong password")
        else:
            messagebox.showerror(title="FATAL ERROR", message="Password not found")
    else:
        messagebox.showerror(title="FATAL ERROR", message="User not found")


def sign_in():
    if u_name.get() != "":
        if pass_var.get() != "":
            for f in forbidden_chars:
                if f in u_name.get():
                    messagebox.showerror(title="FATAL ERROR", message="You can't type these\n"
                                                                      "characters on username:\n"
                                                                      "\ / : * < > |")
                else:
                    if not os.path.exists(u_name.get()):
                        file = open("./" + u_name.get() + "/password.txt", "w")
                        file.write(pass_var.get())
                        file.close()
                    else:
                        messagebox.showerror(title="FATAL ERROR", message="This username already exists!")
        else:
            messagebox.showerror(title="FATAL ERROR", message="Password can't be empty")
    else:
        messagebox.showerror(title="FATAL ERROR", message="Username can't be empty")