I’m trying to collect some good practices and advise for those trying to add i18n support to their themes:

• Read the WordPress documentation on i18n. Print their best practices, put them in a visible place and recite them before any i18n effort.
• Use plenty of format strings. Ok, you already read the best practices, so you known I’m repeating them here. For better format strings use descriptions and comments (the _c() function)
• Use boilerplate copy everywhere. Don’t be too original with the text you include in your theme; “Submit It” better than “Say It Pal!” for a submit button. If your theme includes strings used by most other themes, chances are those strings are already i18n and l10n in . You can always have private copies of your theme with unique headers, titles, and button; the version you share should be as plain and simple as possible.
• Keep out strings that need no translation: and don’t skip small strings subject to translation; alt and title html parameters, submit buttons value, next|prev_post_link and other wp template tags, all accept i18n. See the next two samples; why “Comments” is left out of the _e() function?; is there a translation for ‘|’ in any language?
  

  <?php _e($numComments, ‘XXXXXXX’); ?> Comment(s)

  __(‘ | ‘)

The last and best tip is to use a theme with good i18n support and clean code as base template; sandbox is a great, clean, and fully i18n WP theme.

Working with many development and production some people forgets to check their default charsets; in the end encoding mismatchs crawl. I’ve seen a few programmers falling in this trap.

Last time our php was Latin-1 encoded and the production Apache server used Utf-8 by default. In a shared hosting environments the default encoding can be set from the .htaccess file:

AddCharset ISO-8859-1 .php
AddDefaultCharset ISO-8859-1

Same thing from php:

PHP. Use the header() function before generating any content, e.g.:
header(‘Content-type: text/html; charset=utf-8′);

Links and discussion:

• Basics on http charset parameter at w3c.
• Apache .htacces usage.
• Handy list of IANA accepted encoding names recognized by apache.