Use auto-binding. Make sure that your group that contains the drop down input of languages is in a group that is set to content type of the R.G.
I assume you are creating a list of languages as a Data Type with a Data Field of Language Name. You also want to allow a user to select from a drop down list of all Languages you have already entered into the D.B. for a user to choose from. This user is choosing Languages associated with their user account.
You need in your D.B. a Data Type of User with a Data Field of Language set to a list of Languages (this is your languages from D.B. you have entered as possible choices).
You will have a group that contains your R.G. and the drop down input element. The group will have a content type of user. The data source will be current user.
Your content type of the R.G. will be set to Languages. Your data source will be the Users Languages.
You drop down will use auto-binding to auto-bind to the user’s languages.