Je suis un Formosan.

The name "Formose" for your country is not as well known as "Taïwan" in France, therefore "Formosan" might not be understood (is this a place ? A nationality ? A quality ?).
I would recommend "Je suis Taïwanais.", or if the name "Formose" is really important for you "Je suis de Formose." would work to.

Je vis actuellement dans un petit village.

perfect.
Alternatively : "j'habite dans un petit village.".

Les lucioles peuvent être vues les nuits d'éténuits d'été on peut voir des lucioles.

- Though it wasn't really false (passive construct of sentences is legit), in this context the construction of the sentence wasn't natural.
- les/des : usually "les" is used to refer to a specific thing whereas "des" is for unnamed anonymous or general things. Exemples :
- "Dans mon village il y a *des* chiens sauvages qui trainent dans les rues." : dogs, no tag, no identity, just dogs... being in this sentence just for their quality of being dogs.
- "Les chiens du voisin ont encore hurlé toute la nuit." : the neighbour's dogs, those ones, well identified
- I can't help but remark (don't focus on that for the moment though) :
I think in Chinese you are used to implicit. In French we are usually much more explicit. I guess the purpose of this sentence is to express how remote is your village. We can kind of guess it, but the sentence appears to be drifting on its own in the text. We are used to connect sentences with logical bounds. For instance :
"Comme il est loin de la ville, les nuits d'été on peut voir des lucioles."
"Le village est si éloigné de tout que les nuits d'été on peut voir des lucioles."

J'ai toujours aimé la villcampagne.

- Le village (masculine)
- We wouldn't write "J'ai toujours aimé le village."
we could write "J'ai toujours aimé ce village. " (this village) or "J'ai toujours aimé le village de mes parents. " (the village of my parents)
but what you want to express would be written
"J'ai toujours aimé la campagne." ("campagne" opposes to "city")
"J'ai toujours aimé vivre à la campagne."
"J'ai toujours aimé vivre dans un village."

J'aime passer du temps à l' extérieur.

"à l'extérieur" is OK, although it can simply mean "outside". One could argue that even in the city you can be outside of your flat, but we would understand the feel of "outside the city".
you can alternatively use "J'aime passer du temps dans les champs.", "J'aime passer du temps en forêt.", or more generic : "J'aime passer du temps dans la nature"