In a bilingual area, where the basic knowledge is teached in different languages, there is a dube about the percentage of Spanish that we will use or what are the aspects more relevant to work in one language or another. Some autonomous communities have a certain minimum percentage of L2 that is established in the bilingual area. The learning of subjects must be developed, at least, in a 50% in a foreign language. The value of 50% is the minimum to be met (in communities such as Andalusia), this exposure value may be increased, even up to 100%. This 50% use may be determined in various ways:
Complete units in Spanish and complete units in English. Example: unit 1, 3 and 5 in Spanish; unit 2, 4 and 6 in English.
All units will be teached in both languages, with the teacher choosing which part of the unit to teach in each language. Example: session 1 and 2 in english; session 3 and 4 in spanish.
Establishment of learning situations where students can access resources in both languages depending on the task or challenge posed. It is not doing the same thing with translation in both languages, it is having different activities to develop in one language or another. This way of working must be very well structured to have good results. From my experience, I think it is the most effective for learning the bilingual area.
We will have to apply a participatory and active methodology, with different learning routes that develop the autonomy of the students or teamwork. It should be carefully selected which activities, tasks or learning situations in Spanish or English, depending the intention or the topic to develop in each level.
In the following link you can have and example of planning from a learning situation using Spanish and English Languages.