This is curl with -H "Accept-Language: ru":

And I saw other types of fog without translations.
Thank you for reporting this. It is a known bug: combinations of strings are not translated, even though translation of each particular part exists. It will be fixed
Can I help out with this project?
@Simplyalex99 You mean this particular bug, or the whole wttr.in project?
In both cases the answer is yes.
In the latter case just go through the list of open issues, and select 3-5 issues you like most. After that I will say to you what of these issues is the best choice for beginning.
Прогноз погоды: Saint-Petersburg
Shallow fog
_ - _ - _ - 6 °C
_ - _ - _ → 4 km/h
_ - _ - _ - 6 km
0.0 mm
mm, km, km/h should be мм, км, км/ч
Saint-Petersburg -> Санкт-Петербург
Thank you for the "shallow fog"; I think we should detect untraslated weather conditions automatically, and then translate all of them; agree about the units translation; not agree about Saint-Petersburg:
it always uses the same form in response, as was used in query. I.e, if you would use SpB it would return SpB in the title; if Leningrad, then Leningrad (but not Petrograd, because it is a location in Norway). Thing about it from this perspective: if you query for Phuket, you should receive response for Phuket, and not for จังหวัดภูเก็ต (the latter one you can find in the Location: string though, under the forecast). Theoretically the location name could be translated into the response language too, but it is hard to solve this task in a generalized way (if at all) for all supported languages, all languages in the world, so instead we just use that form in the response that was used in the query
about Saint-Petersburg
agree (:
Freezing rain:
% curl -H "Accept-Language: ru" https://wttr.in/Санкт-Петербург
Прогноз погоды: Санкт-Петербург
.-. Freezing rain
Added translations for Freezing rain and Shallow fog; the bug with the comma-separated sequences is still open
I think that now all of the terms are finally translated, and the bug with comma-separated terms is fixed too. Please post new untranslated strings if you find any (see also #554)