rename g:airline#init#async variable to g:airline#init#vim_async
because that is what it is for: showing whether vim supports async. Is
not set vor nvim, because nvim always supports jobs.
A typical status line for a 'po' (Portable Object) translation file is:
1152 translated messages, 91 fuzzy translations, 42 untranslated messages.
Adding a substitute(), tidies this to:
1152 translated, 91 fuzzy, 42 untranslated
which is still informative, but less verbose.