in the hello example, reorder spanish and french and use variables in switch statement

[dario-piotrowicz]

it's a very minor improvement but I thought I might as well open a PR for it :slightly_smiling_face:

basically this PR does two things in regards to the hello example:

• reorder the places in which french is placed before spanish (to improve consistency, as currently spanish is sometimes before french and sometimes after)
• in the language prefix switch use variables instead of string literals since those are already defined in the guide