feat(i18n): Setup ngx-translate in rxjs-docs

[IraErshova]

Setup ngx-translate with English and Russian languages
Add language service
Create language menu
Add unit tests

[codecov-io]

Codecov Report

Merging #267 into master will increase coverage by 2.76%.
The diff coverage is 90.69%.

@@            Coverage Diff            @@
##           master    #267      +/-   ##
=========================================
+ Coverage   77.14%   79.9%   +2.76%     
=========================================
  Files          15      17       +2     
  Lines         175     214      +39     
  Branches        7       9       +2     
=========================================
+ Hits          135     171      +36     
- Misses         40      41       +1     
- Partials        0       2       +2

Impacted Files	Coverage Δ
src/app/core/data/language.data.ts	100% <100%> (ø)
...c/app/core/components/toolbar/toolbar.component.ts	92.85% <100%> (+7.14%)	⬆️
src/app/app.component.ts	62.06% <83.33%> (+4.37%)	⬆️
src/app/core/services/language.service.ts	89.28% <89.28%> (ø)

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[niklas-wortmann]

Looks pretty good. Thanks for the PR. Just a short question, how is this pr related to #144?

[IraErshova]

@jwo719 It's a replacement for #144

[niklas-wortmann]

Ok get it. Thank you

[ladyleet]

🤗 y'all! this is amazing!

[SangKa]

I'll follow up with the Chinest translation.