[Galaxy S II General] Tiu Fiu:
First of all, welcome to the forum mate... This is going to be a long post, but it is a word of advice worth reading...

Second, coming from a iOS device, I'd like to give you piece of advice: read. And I really, really mean READ. I was lost too when I first came to this forum, back in November 2011, but after a while I found my way around here.

Android gives you A LOT of customizing options, but you should read a bit on the details of Android OS (Android functionality, Android's guts - what are governors, I/O schedulers, what is Under Volting (A.K.A. UV), Over Clocking (OC), etc.) and how to be careful with it - and most of all (probably the one first advice one should give to newcomers), what is the Android Kernel and how it is related to Android OS itself. And what are the differences between the stock Kernel (the one that comes with stock firmwares) and customized Kernels. The reason why I'm saying so much about the Kernel is that it is a very important part of the Android OS, and if you are going to customize your S2, then it is important to now the pros and cons of using custom Kernels (and, more importantly, the DANGERS of flashing Kernels, regardless of being stock or custom ones).

Now, one thing you need to know before flashing anything is that you WILL LOSE YOUR WARRANTY if you modify your OS in any way. That means the slightest customization on anything in the system and Samsung will not think twice before denying any warranty in case you need it.

Stock Firmwares

First things first, if you accepted the risk of losing your warranty, then you should be aware that everything you do here is of your own and absolute risk. Nobody here will be held responsible if you brick your device but yourself (that is one of the general rules around here).

Now, ever since the first ICS for the S2 was released (4.0.3), all languages are included in the ROM it self, including PT-BR (except for Asian firmwares). This means you can flash ANY firmware from ANY european, north-central-and-south american, african, middle-eastern or oceanic country - they WILL have fully functional PT-BR. That being said, forget about Brazilian firmwares - Samsung Brasil has a tendency of only releasing stock firmwares when they become completely obsolete when compared with european firmwares (hell, I remember when ICS was released for Afghanistan and Iraq before it was officially released in Brazil).

Now, when it comes to STOCK firmwares only, you have basically 3 options:

1- Use 4.0.3, which is a very safe, but old firmware.

2- Use 4.0.4, which is a bit more up-to-date than 4.0.3, but has a VERY SEVERE BUG in the stock kernel that CAN HARDBRICK YOUR DEVICE, rendering it unrecoverable (there are ways around this problem).

3- Use 4.1.2, which only has 2 firmwares available: XXLSJ, the leak which has a few annoying bugs, but is quite usable; or the spanish XWLS8, which in my opinion also has its bugs, but it is an official release (by the way, a "leak" is a firmware that was not officially released by Samsung via Kies, but rather obtained by some insider within Samsung's ranks and made available in the internet - thus, it has absolutely NO technical support from Samsung and Samsung cannot be held accountable if anything happens to someone using a leaked firmware)

Custom Firmwares

If you really want to try 4.1.2 (A.K.A. Jelly Bean or JB), I would advise you to learn a bit about custom ROMs. Custom ROMs are modified ROMs created by people all around the forum. They usually consist of stock ROMs which includes several changes to enable modification and/or simply give you a better user experience, but there are also custom ROMs built from scratch (such as CyanogenMod).

The case of 4.1.2 is that it was first leaked in November of last year (if I'm not mistaken) and it was the ONLY stock JB reference we had for almost 3 months - so during that time developers and users around here made their mission to rid that leak from its bugs - and in my opinion they did a fantastic job on that front. And, of course, eventually those fixes were gradually included in the custom ROMs we have around here.

The thing is that there is a LOT of people around here (myself included) that like the stock look and feel of Samsung modified Android (modification called TouchWiz by Samsung itself). So some developers created custom ROMs with the basic stock look and feel of TouchWiz and all the fixes and great things that came along these last 3 months, while giving you THE OPTION of modifying the look and feel IF YOU WANT.

Examples of custom ROMs like that are NeatROM, Omega ROM and AllianceROM - I would suggest them rather than the stock leaked JB or even the stock official JB it self, due to the bug fixes included in them.

Now, I gotta get back to work, so, if you need anything or have any doubts, feel free to ask around here - people are generally friendly and forthcoming on this forum (and specially on this thread), so if you follow the rules and if you are friendly to others, you'll have an opened door here whenever you want.

Best regards and once again apologies for the long post.