What language cards are legal depends on the country. This list is from the Play! Pokemon Rules and Resources document, available
here.
Canada Local—English and French
Europe Local—English, French, German, Italian, Spanish, and Portuguese
Mexico Local—English and Spanish
Russia Local—Russian
South America Local—English, Spanish, and Portuguese
United States, Asia Pacific, and South Africa Local—English
Japan is not listed, but only Japanese cards may be played there.
According to this, Spanish cards are legal only in Europe, Mexico and South America. If you do not live in any of this regions, you cannot play Spanish cards, including Energy. If you do live in these regions, you are free to use cards from whatever languages are legal and may even use cards of multiple languages in your deck.
