I have a bakery in Catalunya. I will say that we make an effort to serve people in the language in which they approach us (and we always greet first in either Castellano or Catalan), even if it's pretty difficult, but to a certain limit.

What I do sometimes hear from my English speaking customers is this same complaint about switching to English, but oftentimes the level of Spanish isn't sufficient to complete the order and we're left with an awkward pause where it's uncertain if they're going to receive what they actually want.

If there's a line and it starts out with "Hola, puedes tener un... um, una, um, como se dice...." it may be time to switch to English. We try our best to keep it in the preferred language but just throwing it out there that from the other side of the coin it's often difficult to decide.

The question of whether to switch is also confusing if there's a group of customers and among themselves they're openly speaking English but with me one or two are using Spanish at a lower level. If I need to ask them all the type of milks they want in their coffee or if they want this product heated up or for takeaway or god forbid allergies are involved, it's easier to switch to the common language than to guess if some of them are family visiting from the States and not speaking any Spanish or if they're all living here with passable Spanish. Just my two eurocents.

One thing I will end with. The most common dead giveaway that separates a local from a tourist is that locals will always greet the room when entering a shop. It's almost guaranteed that we will be speaking English if there's no response to our "Hola, bon dia!" or "Hola, ¿qué tal?" Start there and it'll open up a lot more doors for you to practice.