Pseudo-localization is a test that swaps your source text for an altered version, accented, padded about 40% longer, and wrapped in brackets, so hardcoded strings, truncation, broken fonts, and concatenated sentences show up before any translation exists. Android ships the en-XA and ar-XB pseudo-locales for it.
ICU message format is a syntax for one translatable string that holds variables, plurals, select choices, and ordinals, such as {count, plural, one {# file} other {# files}}. Each language keeps the CLDR plural categories it needs: English uses one and other, while Polish adds few and many.
Hreflang tells Google which URLs are language or regional versions of the same page, so it can show the right one in search results. Add it with HTML link tags, HTTP Link headers, or a sitemap, make every link reciprocal, and use valid codes such as en-GB, not en-UK.
A translation management system (TMS) is software that stores your source content and its translations and runs the translation workflow: translation memory, glossaries, review, machine translation, and integrations. Its translation memory reuses earlier translations by exact or fuzzy match. Crowdin, Lokalise, Phrase, Smartling, and Transifex are common examples.
Use subdirectories such as example.com/de/ when one product shares a site and release process. Only a ccTLD signals a country, subdomains allow separate hosting, Google discourages ?lang= parameters, and it treats vanity domains such as .io and .ai as generic.
Name each language in its own language, such as Deutsch or 日本語, and don't use flags, which represent countries. Link each option to the same page in that language with a crawlable link, remember the choice in a cookie, and never auto-redirect a localized URL.