hardcoded sting "x", should use @string resource

I got this error in my xml file in the layout folder. I changed it to android:text="@string/x" but then it shows this on the buttons:

Here is my main_menu.xml:

<LinearLayout xmlns:android="http://ift.tt/nIICcg"
    android:layout_width="150dp"
    android:layout_height="match_parent"
    android:layout_marginLeft="100dp"
    android:orientation="vertical" >

    <TextView
        android:id="@+id/menu"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Menu" //warning here
        android:textAppearance="?android:attr/textAppearanceLarge" />

    <Button
        android:id="@+id/one_player"
        android:layout_width="115dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="1player" />//warning here

    <Button
        android:id="@+id/two_player"
        android:layout_width="115dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="2players" />//warning here

    <Button
        android:id="@+id/exit_game"
        android:layout_width="wrap_content"
        android:layout_marginTop="20dp"
        android:layout_height="115dp"
        android:text="exitgame" />//warning here

</LinearLayout>