It all depends on how the HTML has been added to the asset.
As sister mentions above, the HTML could be in string tags in the assets config.txt file in which case you can add language specific tags containing the translated texts. For example:-
string-table
{
warning_message_stop "You have passed a stop signal. Apply your brakes immediately"
warning_message_speed "You are speeding. Slow down"
}
You can add the following:-
string-table-fr
{
warning_message_stop "Vous avez passé un signal d'arrêt. Appliquez vos freins immédiatement"
warning_message_speed "Vous accélérez. Ralentir"
}
string-table-nl
{
warning_message_stop "Je hebt een stop-signaal geslaagd. Breng je remmen onmiddellijk aan"
warning_message_speed "Je versnelt. Vertragen"
}
In your case, for German, the tag would be "string-table-de". Trainz will select the language version that matches the language set in Trainz Settings.
The HTML text could also be in a separate HTML asset as .html files. In this case the text can be simply converted into the required target language.
The above are from the Trainz Wiki Page
https://online.ts2009.com/mediaWiki/index.php/How_to_Create_a_HTML_Asset