Multiple translations

[MoRadwan74]

What this PR does

• Add a language selector to toggle the app language according to the available languages.
• Add the Arabic language to the app alongside handling RTL styles.
• Provide a guide in the documentation to show how to add another language easily.

[tephenavies]

Hi @MoRadwan74. This looks great. At the moment we're working on getting community translations available and editable through Weblate: https://hosted.weblate.org/projects/terriajs/terriajs/. I'll revisit this PR late next week.

[MoRadwan74]

Any news? @steve9164

[emiliopardo]

Hello from Spain and sorry for my bad english.

I can cooperate to translate to Spanish (Spain) . I created an acount in weable.org but the translate project is blocked.

[CLAassistant]

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