Hello,
I added mysite/javascript/lang/ru_RU.js and flushed website, but still sapphire/javascript/lang/en_US.js load?

public function init() {
...
Requirements::clear();
}

doesn't help

and as addition how to form structure like that:
<% if Object.Method() == bla-bla %>
<% end_if %>

i know that i can use:
<% control Object %>
<% if Method() == bla-bla %>
<% end_if %>
<% end_contol %>
but it looks odd

I think I'm having similar issue here. Am on SS 2.4.0

Have added $SearchForm in my template and in _config.php:

FulltextSearchable::enable();
Translatable::set_default_locale('nl_NL');
Object::add_extension('SiteTree', 'Translatable');
i18n::set_locale('nl_NL');

Have added following line in mysite/lang/nl_NL.php:

<?php
  $lang['nl_NL']['SearchForm']['SEARCH'] = 'Test';
?>

Now on the site (even with ?locale=nl_NL) the default search form value is taken from saphire/lang/nl_NL.php and not from mysite/lang/nl_NL.php. Newly created $lang rows (i.e. lang definitions not present in saphire/lang/nl_NL.php) in mysite/lang/nl_NL.php do work however.

Is it not possible to overload saphire/lang/nl_NL.php?

PS. This is in my Page.php:

public function init() {
  parent::init();
  // Set locale
  if($this->dataRecord->hasExtension('Translatable')) {
    i18n::set_locale($this->dataRecord->Locale);
  }
...

It's default locale as fallback: sapphire/javascript/i18n.js but u can block it Requirements::block('sapphire/javascript/i18n.js');

Requirements::add_i18n_javascript('<my-module-dir>/javascript/lang');

Fixed this by creating folder zlang in root. Contents of this folder is empty _config.php (with <?php ?>) and lang folder with language files. Think I found this trick on ssbits.