Are certain Japanese words and phrases a regional or an etiquette issue?

はじめまして、みんなさん！
I've been studying Japanese on my own since I was 13 years old.  I've tried various methods from Japanese and anime clubs in school, the internet, books, television, as well as a couple of other Japanese language programs (Talk Now! Instant Immersion, 101 Languages of the World, and a very small bit of Rosetta), however I prefer the method of Rocket.  It's easy to get into, not too monotonous with the information, lets you learn on your own curve, but also lights a fire under you to practice to the best of your ability.  So I am gratefull to this place.

When studying the lessons, one of the things that I learned is that "Welcome" (as in welcome to my home) is "よこそ！(yokoso!)"  However, when I had learned some Japanese on my own prior to picking up Rocket, I learned that welcome is ”Irasshai!”　or "Irasshaimase!"  From my own understanding of Spanish (I took two years in high school and can speak on a conversational level pretty decently), there are some words and phrases such as "asi asi/mas o meno" or "adelante/vamanos" that are more or less regional.  In school I was taught "asi asi", but when conversing with most of my customers in Spanish, I use "mas o meno" since a lot of my clients tell me that asi asi is "archaic".

But I digress.  Is Japanese similar in that certain words and phrases are based on what part of Japan you are in?  Like, if I went to Osaka and used "yokoso", would it be gramatically correct?  And if it is, and then I decide to venture to Tokyo, would yokoso still be proper?  Or would I have to switch over to irasshai?  I've also noticed a similar issue with miruku/gyuunyuu (milk).

Thank you once again Rocket!

Hi Trutenorさん,

Great to hear that Rocket works well for you! :)

The Japanese that is spoken in Tokyo is generally thought of as "normal" Japanese that (almost) everyone understands and can speak, regardless of where they are from. However, there are different dialects spoken in the different regions of Japan where they use certain phrases and words instead of the usual.
E.g. Osaka dialect, Kagoshima dialect, etc.
(Specifically in terms of ようこそ, it would be correct in both Tokyo and Osaka.)

In terms of words that have both a katakana and hiragana word, like ミルク/牛乳(ぎゅうにゅう), either is fine but it might be the case that the people from the older generations may prefer to use the hiragana word because it is the more "traditional" word. Whereas the younger generation may more likely use miruku because it is the more "modern"/English word.

Hope that helps!

Crystal

Thank you so much for your explanation!  That really helps put things into perspective.  You pretty much confirmed the hypothesis that I had.

My main issue was to make sure that I didn't come off as brash and uncouth, since there is already a stereotype against foreigners (Americans being a major offender) not trying to respect the culture and tradition of the land of the Rising Sun.  But on the other hand, I also didn't want to sound "old fashioned" or use Japanese that most people don't use anymore.  I want to be respectful and well mannered, but I also don't want to be stiff like a robot!  I want my personality to shine through!

Once again, thank you for everything!  Your explanation was just like your name.  Clear as Crystal!