D-017 add mandarin

[ghinta]

As a Chinese cat owner I want to to be able to change the language of the app to Chinese so that I can understand the text in the app.

• [x] add spinner to select the language
• [x] add mandarin to the spinner
• [x] add mandarin to the app
• [x] override all system languages

[Toml499]

@Toml499