It would be difficult to comment on a registration requirement in another country. However we don't believe you would need to register in Spain and charge VAT.
If the situation was reversed and you were a Spanish company providing translation services to individuals in the UK, then there would be no requirement to register. There would only be a requirement to register in Spain if you are providing digital services to consumers.
If you are providing digital services then you could register for non union MOSS in any member state and charge and account for the Spanish VAT in this way
The guidance states:
VAT accounting options for UK businesses supplying digital services to consumers in the EU
The place of supply will be where the consumer is located. You must either:
VAT rules for supplies of digital services to consumers
- •register for the Non-Union VAT MOSS scheme in an EU member state
- •register for VAT in each EU member state where you supply digital services to consumers