For residents of an EU country and Switzerland, working in French Guiana is allowed without problem. If you're from outside the EU, you will probably need a work permit - check with the French Embassy in your country. Do not forget though that the unemployment rate is high. But if you work in the health sector doctor, nurse, it will be much easier.
Voluntary service: Volontariat Civil à l'Aide Technique VCAT, (http://www.outre-mer.gouv...). Conditions: you must be French or from another EU-member state or a country belonging to the European Economic Area. You must be over 18 and under 28 years old inclusive. You must not have had your civic rights revoked by a court or have been convicted of certain offences.