To choose the best impersonal "se" phrase that fits with Spain's National Flag, the most appropriate option would be:
se habla
This phrase can be interpreted as "one speaks" or "it is spoken," which could relate to the Spanish language that is associated with Spain. The other options, "se repara" (it is repaired) and "se busca" (one is looking for), do not directly relate to the concept of a national flag.