Large INSERT INTO SELECT [..] FROM gradually gets slower
The large offsets can have this effect. I would try to remove the offset and use only LIMIT 10000:

INSERT INTO domains.dictionary_language
  (text_id, english)
    SELECT      t.old_id,
                IF(t.old_text LIKE '%==English==%', 1, 0)

    FROM        wikt.text AS t
      JOIN      ( SELECT COALESCE(MAX(text_id), 0) AS offset
                  FROM domains.dictionary_language
                ) AS m
                ON  t.old_id &gt; m.offset

    ORDER BY    t.old_id
    LIMIT       100000;