Add language choice for installation

[CodeShakingSheep]

Problem

Nextcloud shows the following warning:

Your installation has no default phone region set. This is required to validate phone numbers in the profile settings without a country code. To allow numbers without a country code, please add "default_phone_region" with the respective ISO 3166-1 code of the region to your config file. For more details see the documentation ↗.

Solution

Added question for language in manifest.toml. Set 3 language variables (see here: https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/config_sample_php_parameters.html#default-language ) in install:

• default_language
• default_locale
• default_phone_region

~~In upgrade I set default_phone_region if it's not set according to default_locale (which always has a value as it has a default value).~~

Edit: Setting default_phone_region for existing installations should be handled in config panel, cf. #638.

PR Status

• [ ] Code finished and ready to be reviewed/tested
• [ ] The fix/enhancement were manually tested (if applicable)

Automatic tests

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