The rules for teaching Spanish as a foreign language in China will be the same as English:

1) Native speaker

2) Batchelors degree

3) probably the Spanish equivalent of a TEFL certificate. 

what requirements did the adverts say?

If you're not suitable or qualified then none of the places you contact will reply to you as its a waste of their time. 

Sorry, the world is divided into English and Chinese only. No other languages spoken anywhere ever.

You need to check the Chinese companies doing business in Panama, they run the canal there and now 20 percent of the population is Chinese. These companies would like to send employees there with some Spanish proficiency because contrary to there talk, they don't hire locals.

I would suggest that you apply directly to universities. The only Spanish teachers that I have met in China all work for universities. They all seem to have applied to the university directly as to going through an agent. Some of them were recruited as Spanish teachers but end up teaching oral English as there is always a shortage of English teachers because of all the rules and regulations that have changed.

Try to send your application to vocational colleges where they teach about 'international business' and also the 'import and export business'. The students there must learn a foreign language they get to pick besides English. Some of the other foreign languages are German and/or Spanish.