informal or formal japanese - which one to focus on?
こんにちは,
I've been wondering if I should focus on informal Japanese rather than formal Japanese.
I study Japanese to understand Japanese shows and movies and also to be able to communicate with the locals fluently when visiting Japan. Should I study formal or informal Japanese? I understand both of them but I'm not sure which one to use since it's kinda hard for me to switch them in different situations.
Let's say, is it rude if I ask a person I don't know a sentence like "最近何があった？" or use "飲んでる" instead of "飲んでます" ?
What is your opinion?

Question

informal or formal japanese - which one to focus on?
こんにちは,
I've been wondering if I should focus on informal Japanese rather than formal Japanese. 
I study Japanese to understand Japanese shows and movies and also to be able to communicate with the locals fluently when visiting Japan. Should I study formal or informal Japanese? I understand both of them but I'm not sure which one to use since it's kinda hard for me to switch them in different situations. 
Let's say, is it rude if I ask a person I don't know a sentence like "最近何があった？" or use "飲んでる" instead of "飲んでます" ?
What is your opinion?

Answer

Doporučuji, abyste se učil formálně. Ale nejprve říkám, "です／ます“ není to formální, ale "obvyklé". Proč je to obvyklé?  Protože já, rodilý Japonce, vždy používám to,  když se poprvé setkávám s dalšími lidmi.

>"also to be able to communicate with the 
>locals fluently when visiting Japan."

Používám "で す / ま す", když mluvím s těmi "místními obyvateli". Kdybych jim řekl "ど う, 元 気?", Mysleli by si "hej, kdo jsi ty? jaký je tvůj jazyk?".

>but I'm not sure which one to use since 
>it's kinda hard for me to switch them in different situations.

Nemusíte je přepínat. Použijte "obvyklé" japonské. To stačí.

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I recommend you should learn formal one.

But first I say, "です／ます“  is not a formal one, but "usual" one.

Why is it usual?  Because I, a native Japanese,  always use it when I meet other persons for the first time.

>"also to be able to communicate with the 
>locals fluently when visiting Japan."

I would use "です／ます” when I talk to those "locals". If I spoke "どう、元気？“ to them, they would think "hey, who are you? what's your language?".

>but I'm not sure which one to use since 
>it's kinda hard for me to switch them in different situations.

You don't have to switch them. Use  "usual" Japanese. That is sufficient.