Some wikis have custom templates like {{User en-2.5}} and so on, while Babel does not support those proficiency levels out of box, this should be fixed.
I can provide a couple of examples based on my own skills. My English is on Upper-Intermediate level. I use {{#babel:en-2}}, but 2 is Intermediate, I am better than that, and sometimes it is an important distinction. Sure enough I can just use {{#babel:en-3}} but then I will be telling untruth, if you give me any Advanced level test I will most probably fail it. Same goes for my Japanese proficiency. I use {{#babel:ja-0}}, well, and as in this case the Babel level provides an option that it is either nothing or a real struggle, it is more fair than with my English. On the other hand, still I'd like to somehow distinguish my a-several-hundred-words-and-basic-grammar Japanese from let's say my absolute-zero Hindi. I still have a way to go to have {{#babel:ja-1}} hanging on my userpage though.
Existing semantics actually differ a bit, https://uk.wikipedia.org/wiki/Template:User_en-2.5 and https://en.wikipedia.org/wiki/Template:User_en-2.5 provide a bit different wording though revolving around the same level, but I do no see it as a problem, as usage of the levels proposed is pretty straightforward.
P.S. Yeah a good way to solve the two dilemmas mentioned is just to improve the languages, but I hope you look at it more generally :)
Complete ILR scale in Babel
Description of levels: https://www.govtilr.org/Skills/ILRscale2.htm
| Level | Description | Cat |
| 0 | This user has no knowledge of English (or understands it with considerable difficulty). | User en-0 |
| 0+ | This user has elementary knowledge of English. | User en-0+ |
| 1 | This user has basic knowledge of English. | User en-1 |
| 1+ | This user has pre-intermediate knowledge of English. | User en-1+ |
| 2 | This user has intermediate knowledge of English. | User en-2 |
| 2+ | This user has upper-intermediate knowledge of English. | User en-2+ |
| 3 | This user has advanced knowledge of English. | User en-3 |
| 3+ | This user has superioir knowledge of English. | User en-3+ |
| 4 | This user has near native speaker knowledge of English. | User en-4 |
| 4+ | This user has profecient knowledge of English. | User en-4+ |
| 5 | This user has professional knowledge of English. | User en-5 |
| N | This user has a native understanding of English. | User en |