Ugh I feel your pain! I'm actually a Brit who moved to NL - so knowing what to get in both English terms and Dutch has been so confusing!
One way to make sure what you're getting is to grow your own herbs. I've got water cress, garden cress and endive seeds from a garden center (marked safe for human consumption too); which are growing indoors. You can also get hold of reptile specific seeds, such as Lucky Reptile which includes things like chickweed, ribwort, yellow clover, dandelions etc. It seems a good mix - at least growing well so far (and I really don't have a green thumb! Simply put them all in a container in a sunny windowsill, and water with osmosis water).
Otherwise pre-done salad bags can be an option. Though I'd be careful that they list what ingredients they say, as of course don't want any with onions or rhubarb - and try to avoid iceberg lettuce; since it's little nutritional value. The terms get so confusing, seems every place pretty much make up their own terms for greens xD I believe the general rule of thumb for lettuce: the darker the leaf, the more nutritional value (of course, can't say for sure how true this is!). As always, just need to make sure it's washed well before use.
Anywho! One we've had success with is 'Lambs Lettuce', which can be found in places like Morrisons. Or as mentioned, water cress is considered to have good nutritional stats. The method I used was to scout out shops (either in person or online) and list out what they specifically had in the packet - such as taking a phone pic of the back of the package to refer to. Probably a bit more extreme than needed, but I also didn't want to risk guessing a translation!
Hopefully a start

- Hons. Degree Animal Management -4 years volunteer in a Zoo -10 years volunteer in animal shelters -Enclosure enthusiast