WordPress database error: Illegal mix of collations

With WCML installed, I have this error showing up in WordPress dashboard plugins page appearing multiple times

WordPress database error: [Illegal mix of collations (utf8_unicode_ci,IMPLICIT) and (utf8_general_ci,IMPLICIT) for operation '=']

as of WCML version 3.5 I will also have this error appearing in every WordPress page (frontend and backend) such as:

SELECT tl.trid AS trid, tl.ttid AS ttid, tl.tlang AS term_lang, tl.pid AS post_id, pl.plang AS post_lang FROM ( SELECT o.object_id AS pid, tt.term_taxonomy_id AS ttid, i.language_code AS tlang, i.trid AS trid FROM wp_term_relationships AS o RIGHT JOIN wp_term_taxonomy AS tt ON o.term_taxonomy_id = tt.term_taxonomy_id LEFT JOIN wp_icl_translations AS i ON i.element_id = tt.term_taxonomy_id AND i.element_type = CONCAT('tax_', tt.taxonomy) WHERE tt.taxonomy = 'product_cat') AS tl LEFT JOIN ( SELECT p.ID AS pid, i.language_code AS plang FROM wp_posts AS p JOIN wp_icl_translations AS i ON i.element_id = p.ID AND i.element_type = CONCAT('post_', p.post_type) ) AS pl ON tl.pid = pl.pid

or

UPDATE wp_term_relationships AS o, wp_icl_translations AS ic, wp_icl_translations AS iw, wp_icl_translations AS ip, wp_posts AS p SET o.term_taxonomy_id = ic.element_id WHERE ic.trid = iw.trid AND ic.element_type = iw.element_type AND iw.element_id = o.term_taxonomy_id AND ic.language_code = ip.language_code AND ip.element_type = CONCAT('post_', p.post_type) AND ip.element_id = p.ID AND o.object_id = p.ID AND iw.element_type = 'tax_product_cat'

I’m not sure why WCML is selecting and updating tables on every page load… Anyway, I had a look at my database and most of my tables are in utf8_general_ci, whereas WPML and WCML tables are in utf_unicode_ci – I tried to convert the collation in one or the other, but the error still appears; I have this issue only with WPML/WCML plugins and when WCML is installed and active – my host is WPEngine. I reckon for a multi-language plugin unicode makes for a better choice when it comes with non-standard non-latin characters, but general_ci is faster.

Perhaps you could adjust the queries and make them compatible? also, do you really need to run multiple mysql update command at every page load? other users reported performance issues: https://wordpress.org/support/topic/very-slow-with-35-update

thank you

https://wordpress.org/plugins/woocommerce-multilingual/