translations: Add more validation, fix many formatting errors

[deltragon]

Fixes #618.
Partially addresses #608.

This PR strongly expands validate_po.py, which now checks the following:

• Translated files have no incorrect placeholders, which would lead to runtime errors.
  There were multiple such cases, including:
  • Unnamed placeholder missing in translation (this was the issue in CI failing due to invalid translations #618)
  • Placeholder in translation broken (eg. % instead of %s, d% instead of %d in RTL languages)
  • Named placeholder in translation that does not exist in original (eg. %(translated_name)s instead of %(name)s)
• Named placeholders are used for plural forms.
  This is not a runtime error on its own - however, many languages translate singular forms with words ("one hour") instead of numbers ("1 hour"). When using unnamed placeholders, this leads to a runtime error again.
• At most one unnamed placeholder is used.
  This means that translations can reorder the sentence as needed.
• The template (.pot) file contains all translatable strings.

Additionally, this PR

• fixes all cases that violated the (old and new) checks
• adds the #, python-format annotation to strings with placeholders, so Weblate can also validate them
• adds a method to extract the translatable strings into the template file (python validate_po.py --extract) - which also adds/preserves the annotation strings
• adds all missing translatable strings to the template, or marks them as not translatable

Once this PR is merged, we need to document the new python validate_po.py --extract helper in our contributing guidelines, so if someone adds a new translation they will know to run the tool.

[archisman-panigrahi]

so if someone adds a new translation they will know to run the tool

@deltragon You are referring to someone adding translation directly via git, bypassing weblate, right?

Also, do you know if the translation you fixed will be automatically sent to weblate? I don't want weblate to automatically overwrite the broken translation you had fixed

[archisman-panigrahi]

I tested that it works. I could not test whether the translations work properly without runtime errors (as my computer's language is English), but I trust your skills.

I approve merging.
Please let me know if this is ready for merging.

[archisman-panigrahi]

• python validate.py --extract

You mean validate_po.py, right?

[deltragon]

You mean validate_po.py, right?

Thanks, corrected the typo.

[deltragon]

You are referring to someone adding translation directly via git, bypassing weblate, right?

No, I mean when someone adds a new translatable string in python or glade. In #569, this was done without adding the strings to the .pot/.po files - with this, they would know what to run to make sure the strings are translatable.

Also, do you know if the translation you fixed will be automatically sent to weblate? I don't want weblate to automatically overwrite the broken translation you had fixed

Yes, they will be - or rather, Weblate will complain about a conflict, if there is one, that can be resolved manually. I have access to pull the changes into Weblate and resolve the conflict if needed.

Please let me know if this is ready for merging.

Thanks! This is ready from my side.

[archisman-panigrahi]

I guess now we need to push from GitHub to Weblate.