Add Internationalization section with gettext-tstrings

[yhay81]

Adds a new Internationalization subsection under Libraries & Frameworks — the list currently has no i18n entry.

gettext-tstrings bridges t-strings to the existing gettext/Babel ecosystem: _(t"Hello {name}") sends the full sentence Hello {name} to a standard PO/MO catalog, translators can reorder or repeat placeholders, and attribute access / calls / translation-side format specs are rejected. Extraction and catalog validation work with unmodified pybabel via entry points. The t-string→msgid convention is specified in a versioned SPEC.md.

Checked against CONTRIBUTING.md:

• No duplicate: searched the list and found no existing i18n/gettext entry (proposed a new category, alphabetically placed between HTML and Utilities, TOC updated).
• Working example, tested on Python 3.14.6:

import gettext
from gettext_tstrings import Translator

_ = Translator(gettext.translation("messages", localedir="locales", languages=["ja"]))
name = "Ada"
print(_(t"Hello {name}"))  # → "Adaさん、こんにちは" (translation may reorder {name})

• Links verified: repo, PyPI (pip install gettext-tstrings, v0.1.0a2), MIT licensed, CI on Linux/macOS/Windows.