Translation:They are building a new mosque in town.
Can this mean a hypothetical/general 'they' in Swedish the way that it can in English, or just a specific one?
It's idiomatic in Swedish to always use the definite for "in town", where English prefers not to. But we do accept "in the town" (and "in the city") as well.
I'm always confused by the é letter. What is it for? Why is it used sometimes?
My guess is that is the accent location. In swedish language accent is on the first syllable, but there are exceptions. Mosqué, ideé...
You're right, but it's not something Swedish words do in general. All (or almost all) such words are French loans that retained the é in Swedish.