The requirements and procedures for naturalisation are broadly similar in all EU countries:
- a minimum period of residence
- language competency
- cultural integration
- knowledge and appreciation of the country’s history
However there are considerable variations in the minimum period of residency (3-10 years), language competency (informal interview, formal examination, age related dispensations), need to give up other nationalities.
N.B. Giving up your British Citizenship will affect your right of abode in the UK. It can also affect things such as wills & inheritance. Before doing so you must obtain specialist advice on how renouncing British Citizenship will affect you, your family & your circumstances.
Of the countries listed below only France allows dual nationality. Germany allows dual nationality for EU Citizens thus the situation is uncertain post-Brexit.
Here is a general document from the European University Institute which summarises the Naturalisation requirements across Europe – http://eudo-citizenship.eu/docs/policy-brief-naturalisation_revised.pdf
Below are references to specific country requirements.