feat: option to change UI language

[VityaSchel]

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Description

Feature request: #1877

This PR adds option to change app's UI language in settings under Appearance tab. I've used native <select> tag and hardcoded all languages as <option> tags. I've used dynamic CSS variables for theming and transition: var(--default-duration) for theme animation.

I've used userConfig which stores data to config.json (uiLanguage property) to save and load language set by user. If language isn't set yet, it falls back to default language, evaluating this: process.env.LANGUAGE || app.getLocale() || 'en'. After choosing language, app is restarted using app.relaunch(); app.exit();

I've added two events to ipc: get-ui-language, set-ui-language.

I've added two translation keys: uiLanguage, uiLanguageDescription.

[KeeJef]

We have some design changes incoming for this one, mainly switching the select tag to the native multi select modal that we use in various places in the Session app (inviting members to communities) and strings changes

[KeeJef]

Tracking internally via SES-2059