If it had not been for my experience with spoken versus written Arabic, I'd be more sympathetic toward the opinions expressed by Bruce.  Arabic truly has one written language and many spoken languages.  Standard English is spoken by everyone I encounter.  Standard English is the language of news reports, documentaries, and university classes.  It is spoken by undereducated people as well as educated ones.

I have taught Englsih to foreigners and one of their first comments is that my English is not the street language taught by some other instructor who feel that students cannot learn the language that they use daily.  Some instructors seem to feel that students will be more comfortable speaking the improper language of other foreigners. 

I recommend ignoring phrasals, not worrying about precise pronunciation, and using synonyms when vocabulary is lacking.  Watch English language documentaries and read nineteenth century British literature.

Much of the recent popular literature uses language that is not suitable for conversation.  Prurient literature sells so publishers push authors to include vulgar language.  American uthors such as John Grisham are good but other authors use language that is unaceptable in public.

For speaking, I'd say phrasal verbs and expressions

Vocabulary, although important, it is not spoken very much unless you want to sound fancy and show off. Some may even be turned off especially in the States if you try to sound too intelligent

If you're interested in literature and writing, vocabulary would be more important

First, to become fluent in conversation, you work toward the most commonly used Nouns and Verbs.There is a list of these called a "Dolch List". You might find the same by a webs search for

"Most Common Nouns" or "Most Common Verbs".

The best study tool that I have ever found for acquiring skill or becoming "fluent" in spoken English is in the study of English songs. The easiest to work with are the slow songs, love ballads and so forth. Some examples of these are in my Notebook Entries. Songs like "Cry Me A River" or

"You've Got A Friend" are basic examples.

Forget about learning "expressions". Those are learned as a matter of course anyway.

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Rene:

 You are asking  about "fluent"  as though you could obtain it  by the abandonment of

 of a Part---of---Speech.   For the context of your question refers to "expressions".

That is not specifically, a Part---of---Speech.

  The point is that you cannot become fluent by the abandonment of anything.

If I understand you correctly  however,  I may add that  "expressions"   are not necessary to learn any language.

    Also, for the sake of practicality, you can consider that there are   2, and not 1  Englishes.

  
#1  Spoken English  for daily use,  will use a   limited vocabulary.

Sentences  like;    "It is time to get up and get ready for work."  or,  

"Let's go get something to eat."  or,   "I really like that!"

   ...are very easy  to learn  for a kind of "fluent"  knowledge.

  #2  If you wish to engage   a    Written English, as it would be undersood for Academic

or  "school study" or professional or business communications,  you will need an expanded vocabulary for  "fluent" expression.