Later Victorian writers connected the practice of sending coded messages through flowers to the ambassador's wife and world traveler Lady Mary Wortley Montague (1689-1762), who wrote to an unnamed English lady in the early 18th century about the concept of a "Turkish love letter" consisting of various objects in a box - a pearl, a clove, a jonquil, a piece of paper, a bit of soap, etc. - each with a specific coded meaning. Her letters were published after her death, but Western references to a language of flowers do not appear until after the turn of the century. Louise Cortambert wrote one of the first books on flower language, La Langage des fleurs, in 1819; it was translated to English within the following decade, and provided the basis for other reference works through the century.

Another early work (Flora's Dictionary, 1829) included, at the end of the preface,

The Lady who has given her leisure hours to this little play of fancy, has not the vanity to attach any serious consequence to it. This bagatelle, she trusts, is too light to attract the grave censure of the critic by profession. It has been an innocent recreation to herself; and it is with no higher expectation than of affording the like amusement to others, that it is now given to the press.

Which certainly implies that in the early years of the tradition at least, it was not taken too seriously as a real method of communication.

However, later in the century, the subject seems to have been given more weight. In the 1882 edition of Gaskell's Compendium of Forms, Educational, Social, Legal and Commercial [...] the Whole Forming a Complete Encyclopedia of Reference, for instance, the language of flowers is taken quite seriously, presented between sections on foreign phrases (generally in Latin or French) and writing elegant/appropriate letters. The compendium is dedicated to "THE YOUNG MEN AND YOUNG WOMEN OF THE UNITED STATES WHO WISH TO MASTER AT THEIR OWN HOMES THE MOST NECESSARY FORMS AND LAWS OF BUSINESS AND SOCIETY": it must have been felt by the compiler that an understanding of floral meanings would be beneficial to someone rising in the middle class, or that leaving descriptions of floral meanings out would cause the work to be taken as incomplete. At the same time, most treatises on decorating with flowers in that part of the period (even in Charles Seelye's Language of Flowers, which gives the meanings of many flowers and bouquets) mainly discuss the appropriateness of certain flowers to the season of the event, or their actual appearances and how they harmonize together, with symbolic meanings the least important aspect of the choice of vegetation. It was important to be aware that flowers could say something and to be able to interpret what someone was telling you in a bouquet, but it was not to be expected that every single floral arrangement was significant in that way.

Can I add on another question... What about fan language, how common was that?

Maybe consider asking a specific person for help from the relevant section of flaired users.

These users look appropriate:

/u/CopperBrook British Politics, Society, and Empire | 1750-Present

/u/zagreus9 British Society and Industry 1750-1914

Not sure how prevalent this was in Iberia and the Continent, but there are some users who might be able to comment on that.

I expect you also searched for previous discussions on the topic?