Translate tooltips and placeholder texts (attributes)

[jelmervdl]

More of a ticket to keep track of this.

Right now element attributes are not translated. Some of these attributes make it into the presentation. The most common:

• <input placeholder="this text">
• <span title="this text">
• <input type="submit" value="this text"> (but that could affect function)

Right now these attributes are going to bergamot-translator, but it doesn't know about their semantics and won't translate them. Since we're already tracking per element what is translated, we could issue translation requests for each of these attributes as well from the extension.