fix: language setter and localstoraging it

src/app/app.component.ts

src/app/components/about-me/about-me.component.html

src/app/components/about-me/about-me.component.scss

src/app/components/global/header/header.component.html

src/app/components/global/header/header.component.scss

src/app/components/global/header/header.component.ts

src/app/components/impressum/impressum.component.html

src/app/components/impressum/impressum.component.scss

src/app/components/impressum/impressum.component.ts

src/assets/i18n/de.json

src/assets/i18n/en.json

src/assets/i18n/fr.json