There are some standards, but they tend to be organization specific (such as DLPT for the US military, or ILR for US Foreign Service Officials), or language specific (such as Japanese Language Proficiency Test).
what taxonomy do they use to separate those who are remarkably fluent from those at various degrees of being less so
In the case of JLPT, the lowest level test (N5) corresponds roughly to a first semester college course in Japanese. The next levels, N4, N3 and N2 correspond roughly to 1 year, 2-3 years and 3-4 years of university level study respectively. If you can pass N1, you can probably do well going to a university in Japan, or work in an office environment in Japan (where everyone else speaks Japanese).
In Europe, you'll find the CERF4 (Common European Framework of Reference for Languages) ratings on many people's CVs or résumés. There are three levels with sub-levels within each.
A. Basic Speaker
A1. Breakthrough or beginner
A2. Waystage or elementary
B. Independent Speaker
B1. Threshold or intermediate
B2. Vantage or upper intermediate
C. Proficient Speaker
C1. Effective Operational Proficiency or advanced
C2. Mastery or proficiency
Stealing shamelessly from wikipedia, here is a description of what is to be expected at each level.
A1 Can understand and use familiar everyday expressions and very basic phrases
aimed at the satisfaction of needs of a concrete type. Can introduce
him/herself and others and can ask and answer questions about personal
details such as where he/she lives, people he/she knows and things
he/she has. Can interact in a simple way provided the other person
talks slowly and clearly and is prepared to help.
A2 Can understand sentences and frequently used expressions related to
areas of most immediate relevance (e.g. very basic personal and family
information, shopping, local geography, employment). Can communicate
in simple and routine tasks requiring a simple and direct exchange of
information on familiar and routine matters. Can describe in simple
terms aspects of his/her background, immediate environment and matters
in areas of immediate need.
B1 Can understand the main points of clear standard input on familiar
matters regularly encountered in work, school, leisure, etc. Can
deal with most situations likely to arise whilst travelling in an
area where the language is spoken. Can produce simple connected
text on topics which are familiar or of personal interest. Can
describe experiences and events, dreams, hopes & ambitions and
briefly give reasons and explanations for opinions and plans.
B2 Can understand the main ideas of complex text on both concrete
and abstract topics, including technical discussions in his/her
field of specialisation. Can interact with a degree of fluency
and spontaneity that makes regular interaction with native
speakers quite possible without strain for either party. Can
produce clear, detailed text on a wide range of subjects and
explain a viewpoint on a topical issue giving the advantages
and disadvantages of various options.
C1 Can understand a wide range of demanding, longer texts, and
recognise implicit meaning. Can express him/herself fluently
and spontaneously without much obvious searching for expressions.
Can use language flexibly and effectively for social, academic
and professional purposes. Can produce clear, well-structured,
detailed text on complex subjects, showing controlled use of
organisational patterns, connectors and cohesive devices.
C2 Can understand with ease virtually everything heard or read.
Can summarise information from different spoken and written
sources, reconstructing arguments and accounts in a coherent
presentation. Can express him/herself spontaneously, very
fluently and precisely, differentiating finer shades of meaning
even in the most complex situations.