Skill Levels for Foreign and Related Languages
by Giampaolo AgostaYou could even just use the skill levels from the Rockhome Gaz as guidelines for how well one speaks a foreign language and related languages:
3-5: Basic understanding, but would likely fail at complex tasks
6-8: fair grounding/apprentice level
9-12: good command/journeyman level
13-15: excellent command/master level
16-17: marvelous command/master level
18+: genius-level
Translated to language proficiency (following the European standards for language proficiency):
3-5: Basic user (A2)
6-8: Independent user (B1)
9-12: Independent user (B2)
13-15: Proficient user (C1)
16-17: Proficient user (C2)
18+: Indistinguishable from a native speaker, can imitate accents
When using a related language, one would work at a fraction of the skill depending on how closely related the languages are -- a reduction of one skill level for closely related languages (e.g., Czech and Slovak), two skill levels for more distantly related languages (e.g., Spanish and Italian), and four levels for distantly related languages (the entire Romance group).
It is obviously a coarse approximation of reality -- in RW some languages are closely related, but not so mutually understandable due to, e.g., pronunciation differences, while in other cases the intelligibility is relatively high in one direction, but not in the other.