United Kingdom
As from 17/05, British people are allowed to travel although with certain restrictions, depending on the traffic-light colour used by the UK to identify the destination country. At present Spain (including the islands) is identified as ‘amber’ which means that before returning to the UK visitors need to get a PCR test as well as booking and paying for two PCR tests for when they are back in the UK, as well as completing the travellers’ form at https://www.gov.uk/provide-journey-contact-details-before-travel-uk .
On return to the UK they are obliged to go into quarantine for 10 days, even if they are vaccinated, and take two PCR test on days 2 and 8 of this period, which must be booked and paid for in advance (see above). They can shorten the quarantine period if they take a private (paid) PCR test on Day 5.
Link to detailed information: https://www.gov.uk/guidance/red-amber-and-green-list-rules-for-entering-england#amber-list
Netherlands
Travellers can leave the country from 19/05 with certain restrictions:
Negative PCR test plus 10 days quarantine when returning to the country of origin if people have visited a high-risk country (which mainland Spain is currently classified as). This condition remains in force until Spain moves to green on the European traffic light system for COVID-19 risk levels.
Information link for Dutch guests: https://www.nederlandwereldwijd.nl/landen/spanje/reizen/reisadvies
Germany
Travellers may leave the country, provided they abide by certain conditions on their return, depending on the contagion risk of the country they have visited.
In the case of Catalonia, which is considered to be a high-risk area, they are obliged to go into quarantine for 10 days on their return and register at https://www.einreiseanmeldung.de.
They can end their quarantine on the fifth day if they take a PCR test and it is negative.
Check here for the latest information, updated on 07/05/2021: https://www.rki.de/DE/Content/InfAZ/N/Neuartiges_Coronavirus/Transport/Archiv_Risikogebiete/Risikogebiete_aktuell_en.pdf?__blob=publicationFile
Belgium
Belgian citizens are obliged to register at https://www.info-coronavirus.be/nl/plf/ to notify their return to Belgium ONLY if they are returning by plane, train or boat. There is no obligation if they are travelling by car.
If they have been outside Belgium for more than 48 hours and are returning by plane, depending on the answers on the official form they must take a PCR test and go into quarantine for 7 days.
If they fail to do so, they risk incurring a fine of €250. https://d34j62pglfm3rr.cloudfront.net/downloads/Internationale_verplaatsingen_07052021_NL.pdf
At present, Catalonia is still classified as red. Consequently, it is likely that Belgians returning by plane who complete the form will be obliged to go into quarantine and take two PCR tests – one on Day 1 and a second one on Day 7 of the quarantine. https://www.info-coronavirus.be/nl/kleurcodes-per-land/
France
The French government imposes certain restrictions depending on the country that has been visited. See: https://www.gouvernement.fr/en/coronavirus-covid-19.
In the case of Spain, as a European Union country, a negative PCR and a self-responsible statement is required whether returning by road, sea or air. If the person cannot present a negative PCR, they must go into mandatory quarantine for 7 days and take a PCR test on arrival: https://www.diplomatie.gouv.fr/en/coming-to-france/coronavirus-advice-for-foreign-nationals-in-france/