I have beginner French and was in France for a week. I noticed in sandwich bars that when choosing what to put on your sandwich people say salade when choosing lettuce. Does salade mean lettuce in this context?
The laitue is indeed a kind of salade, like the mache, roquette, frisée, cresson, pissenlit, batavia, feuille de chêne, scarole, romaine...
Like the English "salad", salade also means any seasoned (etymologically salade means "salted") mixture of food (vegetable, fruit, meat, seafood, cheese, whatever), usually eaten as a starter or for dessert if salade de fruit.
When there is a risk of confusion, we use salade verte to name the former, e.g.:
Il y a de la salade verte dans la salade.
Salade can also be used colloquially to mean any inconsistent, confusing mixture of statements, lies:
Arrête de raconter des salades. - Stop telling bullshit1.
1 Salade is softer than "bullshit" though.
Yes, "salade" can mean "laitue".
"salade" is used to refer to either the raw vegetables themselves (lettuce, laitue), or to the seasoned dish prepared with such vegetables, or even to a dish mixing various ingredients that do not include lettuce, such as "salade de fruit".
"salade" can also be used figuratively to refer to a chaotic situation.