Add language function for multilingual support in sld

A new language function will retrieve the AcceptLanguages parameter value from the request, so that can be used in sld to support multilingual labels.
The sld needs to use the recode transformation function with the language function result as a parameter eg.

<TextSymbolizer>
<Label>
<ogc:Function name="Recode">
<ogc:Function name="language"/>

<ogc:Literal>en</ogc:Literal>
<ogc:Literal>name_en</ogc:Literal>

<ogc:Literal>it</ogc:Literal>
<ogc:Literal>name_it</ogc:Literal>

<ogc:Literal>fr</ogc:Literal>
<ogc:Literal>name_fr</ogc:Literal>
</ogc:Function>
</Label>
<Fill>
<CssParameter name="fill">#000000</CssParameter>
</Fill>
</TextSymbolizer>