The current taxation treaty between Spain and Norway has been in effect since Jan. 1, 2001. But I cannot find an English copy of the treaty.
Skatteavtalen mellom Norge og Spania som trådte i kraft 1.1.2001 innebærer at lønnsinntekt kan tas med ved beskatningen i begge stater, dersom arbeidet er utført i Spania og det er en spansk arbeidsgiver (er det norsk arbeidsgiver og oppholdet er under 183 dager er lønnsinntekten bare skattepliktig til Norge).
Basically it says that Norwegians are taxable in Norway and receive a credit in Norway for income and wealth/capital taxes paid abroad.
Since you are located in Spain, the following form pertains to a Norwegian (resident) employed by a Spanish company, if his/her Spanish residency is shorter than one year (If longer and has taken up residency in Spain: he/she is taxable to Spain)
Skjema for fradrag i norsk skatt for skatt betalt i utlandet (kreditfradrag) for lønnsmottakere, pensjonister og personlig næringsdrivende" (RF-1147)

Form for deducting foreign paid taxes (Credit) from Norwegian taxes for wage earners, pensioners and trade persons.

You find the form RF-1147here
http://w02.signform.com/Scripts/Signform.dll?SignForm&LANG=N...

However, Norwegian translators who work for (not employed by) foreign companies and invoice as proprietary companies (enkeltmannforetak) or limited companies (aksjeselskaper) are only taxable in Norway. Translations are of course reportable as sales and not employment income.
All proprietary and limited companies in Norway are registered at the government's Registry office in Brønnøysund and given a registration number. This number is used for reporting purposes for both taxes and VAT. All companies with sales greater than NOK 35 000 are reporting VAT on domestic sales/purchases, e.g. charging VAT on domestic sales but not exported sales.

[According to an old friend who manages a local tax office in Norway (a ligningssjef). He may be a bit low on the lower VAT limit, but VAT is the responsibility of an other department and does not affect translation jobs for Spanish clients.]