Case of languages

The language specified in a url is case insensitive to google. This google page demonstrates in their example the mixing of cases as seen in the Supported language/region codes section where the codes are Upper Case and in other examples, they are lower case. ISO 639-1 standard specifies to use uppercase for region.

a https://support.google.com/webmasters/answer/189077?hl=en
https://www.iso.org/obp/ui/#iso:code:3166:DE