How to filter a view on a property of optional entityreference-related content (table join / views relationship)?
It seems we cannot use the
  field_data_field_language.field_language_target_id value as a
  condition on the first LEFT JOIN without having another JOIN first,
  which would defeat the purpose - or am I missing something?


Why would adding the column defeat the purpose? Add field_data_field_language.field_language_target_id from your Word and exclude it from the Views display.

Then in your view under Advanced Options add a tag to the query such as concept_word_lang_matrix_view

Then in a custom module implement hook_query_alter() to add your custom join (but not muck with the View fields or anything directly persay -- your Join is just refining results fetched by your View configured in the Views Admin UI)

A sample implementation of hook_query_TAG_alter looks like:

/**
 * Implements hook_query_TAG_alter for the Submission Is Open authentication
 * check of my business logic.
 */
function MYMODULE_query_SubmissionIsOpen_alter(QueryAlterableInterface $query) {
  #dpm(_MYMODULE_get_raw_sql_string($query));
  #dpq($query);
}

/**
 * This is a helper function to output the raw SQL from a db_select() type
 * query.
 *
 * Found via:
 * http://drupal.stackexchange.com/questions/51333/print-the-query-which-is-built-using-db-select
 */
function _MYMODULE_get_raw_sql_string(SelectQueryInterface $query) {
  $string = (string) $query;
  $arguments = $query-&gt;arguments();

  if (!empty($arguments) &amp;&amp; is_array($arguments)) {
    foreach ($arguments as $placeholder =&gt; &amp;$value) {
      if (is_string($value)) {
        $value = "'$value'";
      }
    }
    $string = strtr($string, $arguments);
  }
  return $string;
}


dpm() and dqp() are from the Devel module, they let you preview the SQL running from Views from within your module (where you're tweaking it).

Without having your server/DB I cannot just give you the SQL (nobody can do your homework for you). Writting the correct SQL is the hard part; altering the Views SQL is somewhat tricky but outlined here. goodluck.

EDIT
you may also find this documentation helpful:
https://drupal.org/node/1705998