Mola

(Spanish from Spain)
-Something that's cool, raw
-To really like something or someone

¡Cómo mola este coche!
¿Te mola ese tío?
Me mola tu chaqueta

Botellon

In Spain, when a group or various groups of teens meet at a park or a certain street to drink lots of alcohol (at night, obviously). A group of friends meet in front of a store to buy alcohol, minors stay outside the store while friends who are 18+ buy the drinks, everyone pays their part, but in some cases some people are invited, because she/he is not from the group but a friend of a friend.
Normally everyone brings their own alcohol, some people take sips from other people's glasses/bottles (those are called "gorrones" or "jetas") and some of them don't even drink (they only come to socialize) though it's rare to see.
You can see groups that are separated but some of them go there to meet other groups or to meet new people. If you come to a "botellón" in Spain, don't go by yourself unless you're really good at making friends, because it will be difficult for you to socialize. It will be easier to come with at least one or two friends.
When there's a big festivity in a city like "Halloween", "Carnival", "New Year's Eve", "Pilares" (Zaragoza), "Las Fallas" (Valencia)... parks will be full of people doing "botellón". A group of "botellones" are called a "Macro Botellon".
This concept originated when minors wanted to party and get fucked up but weren't allowed at night clubs. This, of course, is illegal because of the noise, but there are so many people doing this that the police can't control all of them, also when you're caught doing it, you just go somewhere else.

+Vamos a celebrar el fin de curso este Viernes, ¿te vienes de botellón?
-Vale, me apunto ¿a qué hora?

+Ayer no te vi por la Ribera, ¿no te dijeron que había botellon?
-Si que fui, pero me perdí con tanta gente, eso más bien era un Macro Botellón.

No jodas

"are you serious?" in a positive way
do not mistake with "no me jodas" which means the same in a negative way or "don't fuck with me", "leave me alone"
(This exact definition is only applied for Spanish from Spain but it is also used maybe in other context in some other Hispanic countries)

+Nos ha tocado la lotería
-¡No jodas!

+Te han robado la bici
-¡No me jodas!

+Te cortaré el pelo mientras duermes
-Anda tío, no me jodas

tener morro

Literally "morro" means lips (morros) or snout
"Tener morro" is when some people don't pay and the rest of the group have to pay for them all the time, friends who don't buy new clothes and use yours, smoke everyone's weed but never pay, drink from your glass, eat your food...
Normally you tell them "tienes mucho morro" while softly and repeatedly slapping your cheek (like a girl powdering her face).
Some names for these people are: gorrón, caradura, rata, jeta, morrudo, sinvergüenza...
Some other synonims of "tener morro" are: tener un morro que te lo pisas (to have a snout so big you can step on it), tener más cara que espalda (to have more face than back), tener la cara dura (to have a hard face), tener jeta...
Sometimes "Rata" not only means this but also the opposite: to never share or invite other people, like a rat.

+Oye tío, ¿me das de fumar?
- Si hombre, no te voy a dar más que siempre te lo fumas todo y luego no invitas, tú lo que tienes es mucho morro *slapping himself*

+¡Pero qué morro! Deja de comerte mi comida
-No seas rata, que yo siempre comparto contigo
+Anda calla ya y deja de tener morro

Mariachi

Spanish (from Spain) slang for a joint containing both weed (María) and hash (Hachís)

If you invite your partner or friend to a "mariachi" either you'll make their day or they'll get knocked out by it (maybe both)

Maria + Hachís = Mariachi

+Acabo de venir de pillar, ¿nos hacemos un mariachi?
-Venga va